Seamless mobility support and efficient network resource utilization is the key to the performance of mobile multimedia services. Currently, the PMIP (Proxy Mobile-IP) and the IMS (IP Multimedia Subsystem) are considered as the primary means to this end, mostly focusing on unicast applications (e.g., voice over IP). In this paper, we extend the application scope to multicast services and propose a solution framework combining the PMIP with IP multicast. Specifically, we propose an extension of the PMIP (called PMIP-M) for a seamless handover of multicast services that use existing IP multicast protocols, where gradual IP multicast deployment is taken into account for practicality. That is, a user can start or continue to receive multicast service even when he migrates to a network without IP multicast capability. The performance of the proposed scheme is analyzed by extensive simulations.