diff --git a/src/main/java/de/thetaphi/forbiddenapis/Checker.java b/src/main/java/de/thetaphi/forbiddenapis/Checker.java
index cb90abc5..83b200e4 100644
--- a/src/main/java/de/thetaphi/forbiddenapis/Checker.java
+++ b/src/main/java/de/thetaphi/forbiddenapis/Checker.java
@@ -444,8 +444,11 @@ boolean checkType(Type type) {
         while (type != null) {
           switch (type.getSort()) {
             case Type.OBJECT:
-              // don't check superclasses (TODO: investigate):
-              return checkClassUse(type.getInternalName());
+              if (checkClassUse(type.getInternalName())) {
+                return true;
+              }
+              final ClassSignatureLookup c = lookupRelatedClass(type.getInternalName());
+              return (c != null && checkClassDefinition(c.superName, c.interfaces));
             case Type.ARRAY:
               type = type.getElementType();
               break;
diff --git a/src/test/antunit/Java5ClassReferences.class b/src/test/antunit/Java5ClassReferences.class
index 8ecd75b7..9a254cfd 100644
Binary files a/src/test/antunit/Java5ClassReferences.class and b/src/test/antunit/Java5ClassReferences.class differ
diff --git a/src/test/antunit/Java5ClassReferences.java b/src/test/antunit/Java5ClassReferences.java
index de93421a..c2db010b 100644
--- a/src/test/antunit/Java5ClassReferences.java
+++ b/src/test/antunit/Java5ClassReferences.java
@@ -17,6 +17,8 @@
 /* The binary class file is packaged together with the source distribution.
  */
 
+import java.util.*;
+
 class Java5ClassReferences {
   static Integer[][] test() {
     Integer.class.getName();
@@ -28,4 +30,9 @@ static Integer[][] test() {
   
   private Integer field1;
   private final Integer[] field2 = null;
+  
+  // we forbid the superclass or interface, but concrete instance is used:
+  public List<?> list1;
+  public ArrayList<?> list2;
+  public HashSet<?> set1;
 }
diff --git a/src/test/antunit/TestClassReferences.xml b/src/test/antunit/TestClassReferences.xml
index 7b82ef8e..92d5fd5b 100644
--- a/src/test/antunit/TestClassReferences.xml
+++ b/src/test/antunit/TestClassReferences.xml
@@ -21,10 +21,13 @@
       <forbiddenapis>
         <fileset file="Java5ClassReferences.class"/>
         java.lang.Integer @ Forbidden class reference!
+        java.util.List @ Forbidden class reference!
+        java.util.AbstractCollection @ Forbidden class reference!
+        java.util.AbstractSet @ Forbidden class reference!
       </forbiddenapis>
     </au:expectfailure>
     <au:assertLogContains level="error" text="java.lang.Integer [Forbidden class reference!]"/> 
-    <au:assertLogContains level="error" text=" 8 error(s)"/> 
+    <au:assertLogContains level="error" text=" 11 error(s)"/> 
   </target>
   
 </project>
\ No newline at end of file