diff --git a/build.xml b/build.xml
index a3dcb5e..1f8be01 100644
--- a/build.xml
+++ b/build.xml
@@ -113,9 +113,20 @@
 
   <!-- optimizes, combines and removes dead code -->
   <target name="proguard" depends="dist">
+    <!-- figure out where the Java rt.jar file lives -->
+    <if><and><os family="mac"/><os family="unix"/></and><then>
+      <property name="rtClasses" value="${java.home}/../Classes/classes.jar"/>
+    </then><else>
+      <property name="rtClasses" value="${java.home}/lib/rt.jar"/>
+    </else></if>
+
+    <mkdir dir="${deploy.dir}"/>
+
     <taskdef resource="proguard/ant/task.properties"
              classpath="${deploy.dir}/lib/proguard.jar"/>
-    <proguard configuration="etc/getdown.pro"/>
+    <proguard configuration="etc/getdown.pro">
+      <libraryjar name="${rtClasses}"/>
+    </proguard>
 
     <taskdef name="weave" classname="com.rc.retroweaver.ant.RetroWeaverTask"
              classpath="${deploy.dir}/lib/retroweaver-all-1.2.2.jar"/>
diff --git a/etc/getdown.pro b/etc/getdown.pro
index 63788fc..56094ff 100644
--- a/etc/getdown.pro
+++ b/etc/getdown.pro
@@ -14,7 +14,6 @@
 -injars dist/lib/commons-io.jar(!META-INF/*)
 -injars dist/lib/snark.jar(!META-INF/*)
 
--libraryjars <java.home>/lib/rt.jar
 -dontskipnonpubliclibraryclasses
 
 -outjars dist/getdown-pro.jar