IP multicasting is an efficient group communication mechanism. It avoids transmitting packets from a sender to each of the receivers separately. With the current IP multicast mechanism, once having joined a group, a host will receive all data destined to the group from any source, irrespective of whether it wishes to receive them. This paper studies source filtering in IP multicasting. Source filtering is the ability for an individual host to specify the reception of packets sent to a multicast group only from a list of source addresses or to explicitly identify a list of the sources whose data the host does not want to receive. We investigate the issue of source filtering in the context of multicast routing protocols, and propose a solution to support source filtering in IP multicast routing. We have also conducted simulations to evaluate the performance of the proposed mechanism. The results show that, compared to multicasting without the capability of source filtering, our mechanism allows better bandwidth utilization and scalability, thus achieving a truly efficient use of resources for IP multicasting.