diff --git a/bitcoin/messages.py b/bitcoin/messages.py
index 59bd55cc..b295e71f 100644
--- a/bitcoin/messages.py
+++ b/bitcoin/messages.py
@@ -51,11 +51,13 @@ def msg_ser(self, f):
     def msg_deser(cls, f, protover=PROTO_VERSION):
         raise NotImplementedError
 
-    def to_bytes(self):
+    def to_bytes(self, params=None):
+        if params is None:
+            params = bitcoin.params
         f = _BytesIO()
         self.msg_ser(f)
         body = f.getvalue()
-        res = bitcoin.params.MESSAGE_START
+        res = params.MESSAGE_START
         res += self.command
         res += b"\x00" * (12 - len(self.command))
         res += struct.pack(b"<I", len(body))
@@ -74,13 +76,15 @@ def from_bytes(cls, b, protover=PROTO_VERSION):
         return MsgSerializable.stream_deserialize(f, protover=protover)
 
     @classmethod
-    def stream_deserialize(cls, f, protover=PROTO_VERSION):
+    def stream_deserialize(cls, f, params=None, protover=PROTO_VERSION):
+        if params is None:
+            params = bitcoin.params
         recvbuf = ser_read(f, 4 + 12 + 4 + 4)
 
         # check magic
-        if recvbuf[:4] != bitcoin.params.MESSAGE_START:
+        if recvbuf[:4] != params.MESSAGE_START:
             raise ValueError("Invalid message start '%s', expected '%s'" %
-                             (b2x(recvbuf[:4]), b2x(bitcoin.params.MESSAGE_START)))
+                             (b2x(recvbuf[:4]), b2x(params.MESSAGE_START)))
 
         # remaining header fields: command, msg length, checksum
         command = recvbuf[4:4+12].split(b"\x00", 1)[0]
diff --git a/bitcoin/tests/test_params.py b/bitcoin/tests/test_params.py
new file mode 100644
index 00000000..83b90e1c
--- /dev/null
+++ b/bitcoin/tests/test_params.py
@@ -0,0 +1,35 @@
+# Copyright (C) 2013-2014 The python-bitcoinlib developers
+#
+# This file is part of python-bitcoinlib.
+#
+# It is subject to the license terms in the LICENSE file found in the top-level
+# directory of this distribution.
+#
+# No part of python-bitcoinlib, including this file, may be copied, modified,
+# propagated, or distributed except according to the terms contained in the
+# LICENSE file.
+
+import unittest
+import bitcoin
+
+from bitcoin.messages import msg_addr
+
+class Test_params(unittest.TestCase):
+    def tearDown(self):
+        bitcoin.SelectParams('mainnet')
+
+    def test_mainnet_magic_byte(self):
+        bitcoin.SelectParams('mainnet')
+        self.assertEquals(bitcoin.MainParams().MESSAGE_START, msg_addr().to_bytes()[0:4])
+
+    def test_testnet_magic_byte(self):
+        bitcoin.SelectParams('testnet')
+        self.assertEquals(bitcoin.TestNetParams().MESSAGE_START, msg_addr().to_bytes()[0:4])
+
+    def test_mainnet_params_magic_byte(self):
+        bitcoin.SelectParams('testnet')
+        self.assertEquals(bitcoin.MainParams().MESSAGE_START, msg_addr().to_bytes(params=bitcoin.MainParams())[0:4])
+
+    def test_testnet_params_magic_byte(self):
+        bitcoin.SelectParams('mainnet')
+        self.assertEquals(bitcoin.TestNetParams().MESSAGE_START, msg_addr().to_bytes(params=bitcoin.TestNetParams())[0:4])