The Globus Toolkit is an open source software toolkit that has become the de-facto standard for middleware used to build Grid services. Globus XIO is an eXtensible Input/Output library for Globus. It provides a POSIX-like API to swappable I/O implementations - essentially "I/O plugins" for Globus. TCP-XM is a modified TCP engine that supports multicast transmission. We have taken this protocol and wrapped it using XIO to extend the Globus Toolkit to support multicast transmission. This paper describes the implementation and operation of our Globus XIO multicast transport driver. It provides an overview of the TCP-XM protocol design, and describes some initial experimental results.