diff --git a/conntrackt/iptables.py b/conntrackt/iptables.py
--- a/conntrackt/iptables.py
+++ b/conntrackt/iptables.py
@@ -31,12 +31,12 @@ class Rule(object):
         produce comments in the output.
         """
 
-        if protocol not in ("TCP", "UDP", "ICMP"):
+        if protocol.upper() not in ("TCP", "UDP", "ICMP"):
             raise ValueError("Unsupported protocol specified: %s" % protocol)
 
         self.source = source
         self.destination = destination
-        self.protocol = protocol
+        self.protocol = protocol.lower()
         self.port = port
         self.description = description