diff --git a/gr-jay/grc/jay_message_filter.xml b/gr-jay/grc/jay_message_filter.xml
index ddcf3d4..f6afa47 100644
--- a/gr-jay/grc/jay_message_filter.xml
+++ b/gr-jay/grc/jay_message_filter.xml
@@ -4,13 +4,24 @@
   <key>jay_message_filter</key>
   <category>[jay]</category>
   <import>import jay</import>
-  <make>jay.message_filter()</make>
+  <make>jay.message_filter($filterString, $substString)</make>
+  <param>
+    <name>Filter String</name>
+    <key>filterString</key>
+    <value>freq_rel_Hz</value>
+    <type>string</type>
+  </param>
+  <param>
+    <name>Substitute String</name>
+    <key>substString</key>
+    <value>freq</value>
+    <type>string</type>
+  </param>
    <sink>
     <name>in</name>
     <type>message</type>
     <optional>1</optional>
   </sink>
-
   <source>
     <name>out</name>
     <type>message</type>
diff --git a/gr-jay/python/message_filter.py b/gr-jay/python/message_filter.py
index 888d2ee..2d25865 100644
--- a/gr-jay/python/message_filter.py
+++ b/gr-jay/python/message_filter.py
@@ -27,13 +27,14 @@ class message_filter(gr.basic_block):
     """
     docstring for block message_filter
     """
-    def __init__(self, filter='freq'):
+    def __init__(self, filter, subst=''):
         gr.basic_block.__init__(self,
             name="message_filter",
             in_sig=None,
             out_sig=None)
 
         self.filter = filter
+        self.subst = subst
 
         # Register in / out message ports
         self.port_in = pmt.intern("in")
@@ -47,9 +48,13 @@ class message_filter(gr.basic_block):
 
     def delegate_pair(self, pair):
         key = pmt.car(pair)
+        val = pmt.cdr(pair)
         key_str = pmt.symbol_to_string(key)
 
         if self.filter in key_str:
+            if len(self.subst):
+                pair = pmt.cons(pmt.string_to_symbol(self.subst), val)
+
             self.message_port_pub(self.port_out, pair)
 
     def msg_handler(self, msg):