diff --git a/pubsub/iterator.go b/pubsub/iterator.go
index 8f1d931..da888ad 100644
--- a/pubsub/iterator.go
+++ b/pubsub/iterator.go
@@ -218,14 +218,15 @@
 	it.mu.Lock()
 	now := time.Now()
 	for _, m := range msgs {
-		m.receiveTime = now
-		addRecv(m.ID, m.ackID, now)
-		m.doneFunc = it.done
-		it.keepAliveDeadlines[m.ackID] = maxExt
+		ackh, _ := m.ackHandler()
+		ackh.receiveTime = now
+		addRecv(m.ID, ackh.ackID, now)
+		ackh.doneFunc = it.done
+		it.keepAliveDeadlines[ackh.ackID] = maxExt
 		// Don't change the mod-ack if the message is going to be nacked. This is
 		// possible if there are retries.
-		if !it.pendingNacks[m.ackID] {
-			ackIDs[m.ackID] = true
+		if !it.pendingNacks[ackh.ackID] {
+			ackIDs[ackh.ackID] = true
 		}
 	}
 	deadline := it.ackDeadline()
diff --git a/pubsub/message.go b/pubsub/message.go
index 00076ef..54be363 100644
--- a/pubsub/message.go
+++ b/pubsub/message.go
@@ -36,18 +36,12 @@
 	// labelled with.
 	Attributes map[string]string
 
-	// ackID is the identifier to acknowledge this message.
-	ackID string
-
 	// PublishTime is the time at which the message was published. This is
 	// populated by the server for Messages obtained from a subscription.
 	//
 	// This field is read-only.
 	PublishTime time.Time
 
-	// receiveTime is the time the message was received by the client.
-	receiveTime time.Time
-
 	// DeliveryAttempt is the number of times a message has been delivered.
 	// This is part of the dead lettering feature that forwards messages that
 	// fail to be processed (from nack/ack deadline timeout) to a dead letter topic.
@@ -59,19 +53,23 @@
 	// size is the approximate size of the message's data and attributes.
 	size int
 
-	calledDone bool
-
-	// The done method of the iterator that created this Message.
-	doneFunc func(string, bool, time.Time)
-
 	// OrderingKey identifies related messages for which publish order should
 	// be respected. If empty string is used, message will be sent unordered.
 	OrderingKey string
+
+	// ackh handles Ack() or Nack().
+	ackh ackHandler
+}
+
+// NewMessage creates a message with a custom ack/nack handler, which should not
+// be nil.
+func NewMessage(ackh ackHandler) *Message {
+	return &Message{ackh: ackh}
 }
 
 func toMessage(resp *pb.ReceivedMessage) (*Message, error) {
 	if resp.Message == nil {
-		return &Message{ackID: resp.AckId}, nil
+		return &Message{ackh: &psAckHandler{ackID: resp.AckId}}, nil
 	}
 
 	pubTime, err := ptypes.Timestamp(resp.Message.PublishTime)
@@ -86,13 +84,13 @@
 	}
 
 	return &Message{
-		ackID:           resp.AckId,
 		Data:            resp.Message.Data,
 		Attributes:      resp.Message.Attributes,
 		ID:              resp.Message.MessageId,
 		PublishTime:     pubTime,
 		DeliveryAttempt: deliveryAttempt,
 		OrderingKey:     resp.Message.OrderingKey,
+		ackh:            &psAckHandler{ackID: resp.AckId},
 	}, nil
 }
 
@@ -102,7 +100,9 @@
 // Client code must call Ack or Nack when finished for each received Message.
 // Calls to Ack or Nack have no effect after the first call.
 func (m *Message) Ack() {
-	m.done(true)
+	if m.ackh != nil {
+		m.ackh.OnAck()
+	}
 }
 
 // Nack indicates that the client will not or cannot process a Message passed to the Subscriber.Receive callback.
@@ -111,15 +111,58 @@
 // Client code must call Ack or Nack when finished for each received Message.
 // Calls to Ack or Nack have no effect after the first call.
 func (m *Message) Nack() {
-	m.done(false)
+	if m.ackh != nil {
+		m.ackh.OnNack()
+	}
 }
 
-func (m *Message) done(ack bool) {
-	if m.calledDone {
+// ackHandler performs a safe cast of the message's ack handler to psAckHandler.
+func (m *Message) ackHandler() (*psAckHandler, bool) {
+	ackh, ok := m.ackh.(*psAckHandler)
+	return ackh, ok
+}
+
+func (m *Message) ackID() string {
+	if ackh, ok := m.ackh.(*psAckHandler); ok {
+		return ackh.ackID
+	}
+	return ""
+}
+
+// ackHandler implements ack/nack handling.
+type ackHandler interface {
+	OnAck()
+	OnNack()
+}
+
+// psAckHandler handles ack/nack for the pubsub package.
+type psAckHandler struct {
+	// ackID is the identifier to acknowledge this message.
+	ackID string
+
+	// receiveTime is the time the message was received by the client.
+	receiveTime time.Time
+
+	calledDone bool
+
+	// The done method of the iterator that created this Message.
+	doneFunc func(string, bool, time.Time)
+}
+
+func (ah *psAckHandler) OnAck() {
+	ah.done(true)
+}
+
+func (ah *psAckHandler) OnNack() {
+	ah.done(false)
+}
+
+func (ah *psAckHandler) done(ack bool) {
+	if ah.calledDone {
 		return
 	}
-	m.calledDone = true
-	if m.doneFunc != nil {
-		m.doneFunc(m.ackID, ack, m.receiveTime)
+	ah.calledDone = true
+	if ah.doneFunc != nil {
+		ah.doneFunc(ah.ackID, ack, ah.receiveTime)
 	}
 }
diff --git a/pubsub/streaming_pull_test.go b/pubsub/streaming_pull_test.go
index ba40a2d..e70d2e7 100644
--- a/pubsub/streaming_pull_test.go
+++ b/pubsub/streaming_pull_test.go
@@ -67,7 +67,7 @@
 func testStreamingPullIteration(t *testing.T, client *Client, server *mockServer, msgs []*pb.ReceivedMessage) {
 	sub := client.Subscription("S")
 	gotMsgs, err := pullN(context.Background(), sub, len(msgs), func(_ context.Context, m *Message) {
-		id, err := strconv.Atoi(m.ackID)
+		id, err := strconv.Atoi(m.ackID())
 		if err != nil {
 			panic(err)
 		}
@@ -83,20 +83,21 @@
 	}
 	gotMap := map[string]*Message{}
 	for _, m := range gotMsgs {
-		gotMap[m.ackID] = m
+		gotMap[m.ackID()] = m
 	}
 	for i, msg := range msgs {
 		want, err := toMessage(msg)
 		if err != nil {
 			t.Fatal(err)
 		}
-		want.calledDone = true
-		got := gotMap[want.ackID]
+		wantAckh, _ := want.ackHandler()
+		wantAckh.calledDone = true
+		got := gotMap[wantAckh.ackID]
 		if got == nil {
-			t.Errorf("%d: no message for ackID %q", i, want.ackID)
+			t.Errorf("%d: no message for ackID %q", i, wantAckh.ackID)
 			continue
 		}
-		if !testutil.Equal(got, want, cmp.AllowUnexported(Message{}), cmpopts.IgnoreTypes(time.Time{}, func(string, bool, time.Time) {})) {
+		if !testutil.Equal(got, want, cmp.AllowUnexported(Message{}, psAckHandler{}), cmpopts.IgnoreTypes(time.Time{}, func(string, bool, time.Time) {})) {
 			t.Errorf("%d: got\n%#v\nwant\n%#v", i, got, want)
 		}
 	}
@@ -235,10 +236,10 @@
 	}
 	seen := map[string]bool{}
 	for _, gm := range gotMsgs {
-		if seen[gm.ackID] {
-			t.Fatalf("duplicate ID %q", gm.ackID)
+		if seen[gm.ackID()] {
+			t.Fatalf("duplicate ID %q", gm.ackID())
 		}
-		seen[gm.ackID] = true
+		seen[gm.ackID()] = true
 	}
 	if len(seen) != nMessages {
 		t.Fatalf("got %d messages, want %d", len(seen), nMessages)
diff --git a/pubsub/subscription.go b/pubsub/subscription.go
index 59f296e..7e5f201 100644
--- a/pubsub/subscription.go
+++ b/pubsub/subscription.go
@@ -910,9 +910,10 @@
 						// Return nil if the context is done, not err.
 						return nil
 					}
-					old := msg.doneFunc
+					ackh, _ := msg.ackHandler()
+					old := ackh.doneFunc
 					msgLen := len(msg.Data)
-					msg.doneFunc = func(ackID string, ack bool, receiveTime time.Time) {
+					ackh.doneFunc = func(ackID string, ack bool, receiveTime time.Time) {
 						defer fc.release(msgLen)
 						old(ackID, ack, receiveTime)
 					}
diff --git a/pubsub/topic.go b/pubsub/topic.go
index 07e392e..1dd50c0 100644
--- a/pubsub/topic.go
+++ b/pubsub/topic.go
@@ -470,6 +470,13 @@
 	err      error
 }
 
+// NewPublishResult returns the set() function to enable callers from outside
+// this package to store and call it (e.g. unit tests).
+func NewPublishResult() (*PublishResult, func(string, error)) {
+	result := &PublishResult{ready: make(chan struct{})}
+	return result, result.set
+}
+
 // Ready returns a channel that is closed when the result is ready.
 // When the Ready channel is closed, Get is guaranteed not to block.
 func (r *PublishResult) Ready() <-chan struct{} { return r.ready }