diff --git a/spring-integration-core/src/main/java/org/springframework/integration/store/MessageGroup.java b/spring-integration-core/src/main/java/org/springframework/integration/store/MessageGroup.java
index de78c69724..47deceac8a 100644
--- a/spring-integration-core/src/main/java/org/springframework/integration/store/MessageGroup.java
+++ b/spring-integration-core/src/main/java/org/springframework/integration/store/MessageGroup.java
@@ -50,12 +50,6 @@ public interface MessageGroup {
 	 */
 	int size();
 
-	/**
-	 * Mark all unmarked messages in the group. A MessageGroupProcessor typically invokes this method after
-	 * processing all unmarked messages.
-	 */
-	void markAll();
-
 	/**
 	 * @return a single message from the group
 	 */
diff --git a/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageGroup.java b/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageGroup.java
index 776d956393..0700e67625 100644
--- a/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageGroup.java
+++ b/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageGroup.java
@@ -115,11 +115,15 @@ public class SimpleMessageGroup implements MessageGroup {
 	}
 
 	public Collection<Message<?>> getUnmarked() {
-		return Collections.unmodifiableCollection(unmarked);
+		synchronized (lock) {
+			return Collections.unmodifiableCollection(unmarked);
+		}
 	}
 
 	public Collection<Message<?>> getMarked() {
-		return Collections.unmodifiableCollection(marked);
+		synchronized (lock) {
+			return Collections.unmodifiableCollection(marked);
+		}
 	}
 
 	public Object getCorrelationKey() {
diff --git a/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageStore.java b/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageStore.java
index 4d3073a101..5831fd742b 100644
--- a/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageStore.java
+++ b/spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageStore.java
@@ -115,10 +115,9 @@ public class SimpleMessageStore extends AbstractMessageGroupStore implements Mes
 
 	public MessageGroup markMessageGroup(MessageGroup group) {
 		Object correlationId = group.getCorrelationKey();
-		MessageGroup internal = getMessageGroupInternal(correlationId);
+		SimpleMessageGroup internal = getMessageGroupInternal(correlationId);
 		internal.markAll();
-		group.markAll();
-		return group;
+		return internal;
 	}
 
 	public void removeMessageGroup(Object correlationId) {