Many group-oriented applications require authenticating the source of the received traffic, such as broadcasting stock quotes and video-conferencing and hence source authentication is an important component in the whole multicast security architecture. Multicast source authentication must take into consideration the scalability and the efficiency of the underlying cryptographic schemes and mechanisms, because multicast groups can be very large and the exchanged data is likely to be important in volume (streaming). Besides, multicast source authentication must be robust enough against packet loss because most of multicast multimedia applications do not use reliable packet delivery. We propose a new adaptive and efficient source authentication protocol which tolerates packet loss and guarantees nonrepudiation for multicast flows. We have simulated our protocol using NS-2, and the simulation results show that the protocol has remarkable features and efficiency compared to other recent source authentication protocols.