In ATM networks, various usage parameter control (UPC) strategies are used to regulate incoming traffic according to the characteristics of the sources declared at call setup. Among these UPC schemes, the leaky bucket method is well known and has been discussed extensively. The authors study the characteristics of the multiplexer in which cell arrivals are first policed by enhanced leaky buckets (ELBs) to police mean and peak rates simultaneously. Based on the criteria of delay experienced in the buffer, they derive the upper and lower delay bounds expressed as a function of the ELB parameters and the number of sources. They also rigorously prove that these bounds occur with respect to a coincident environment and a perfectly interleaved environment, respectively.