A well accepted policing mechanism for ATM networks is the leaky bucket mechanism. The original leaky bucket has been shown to be effective for traffic with a constant bit rate, but reacts poorly to bursty traffic. Although there are some other policing mechanisms in the literature that provide improvements over the original leaky bucket, most of them enforce the negotiated parameters for individual source traffic and do not fully utilize the statistical multiplexing of multiple connections. As a result, the bandwidth utilisation is low. Even though some source policing mechanisms have been proposed for group connections, they either fail to explore statistical multiplexing fully or do not provide a scheme to protect well-behaved sources from malicious ones in the group. We propose a new source policing mechanism, called the token-bank leaky bucket, for group connections in ATM networks. The mechanism explores the statistical multiplexing of multiple connections in the group and allows the unused bandwidth to be effectively shared by the connections that need it. In addition, it sets a limit of excessive data cells a source can send in a cycle in order to protect well-behaved sources against malicious sources. It is shown by simulations to have a lower violation probability and better bandwidth utilization even for small group connections when compared to the original leaky bucket with a similar configuration.