New high speed networks and the requirements of users for the support of interpersonal communication in a computerised heterogeneous environment demand for new communication services. These services have to provide support for multimedia group communication. In this paper we introduce a multicast transport service for multimedia group applications called M-Connection Service. It is a protocol independent service which provides an interface to different transport protocols. Apart from data transfer this service offers functions to establish, join, leave, change properties of and terminate multicast connections. Multicast connections are characterised by QoS and integrity conditions. We propose distinct QoS negotiation and group management schemes suitable for multicast multimedia communication. We describe the service elements of the M-Connection Service in detail. To demonstrate the feasibility of the service its service elements are mapped onto XTP 4.0 protocol mechanisms.