IP multicast is increasingly used as an efficient communication mechanism for group-oriented applications on the Internet. This success urged the development of security mechanisms for that communication model. A common drawback of the proposed solutions is that they do not take into consideration the dynamicity of group members. This leads to inefficient solutions for real multicast sessions. In this paper, we first classify proposed protocols for secure multicast and point out their non-suitability for dynamic groups. We then propose an efficient protocol, called AKMP, which maintains good performance by adapting the key management process to the membership frequency during the multicast session. Simulation results show that our protocol is more efficient than existing protocols.