We have previously reported a design for a host interface board intended to connect workstations to ATM networks, and an implementation that was underway. Since then, we have made some modifications to the hardware implementation, and implemented software support. Our prototype connects an IBM RS/6000 to a SONET-based ATM network carrying data at the OC-3c rate of 155Mbps. In this paper, we discuss an experimental evaluation of the interface and supporting software. Our experiments uncovered an unexpected bottleneck in providing high bandwidth to application processes, and we suggest a number of possible improvements to workstation architectures to address this bottleneck.