The need for an airborne system to alert flight crew members to potential collision hazards has been recognized since the beginning of the “jet age.” The search for a technically feasible system started in earnest in 1955. The most recent version, the present traffic Alert Collision Avoidance System (TCAS II) is the only one that has achieved serious consideration for implementation in the National Airspace System. In support of this evaluation process, the current study was performed to compare the effectiveness of different TCAS display formats on flight crew workload. Sixteen Boeing-727 flight crews from eleven different U.S. airlines participated in the study. The crews flew a Phase it certified Link/Boeing 727 simulator in a simulated Air Traffic Control radar environment. Eight flight legs were flown by each crew using one of the four TCAS configurations tested: (1) no TCAS, (2) limited TCAS, (3) TCAS with a target-activated display, and (4) TCAS with a full-time display. Workload ratings were collected after each flight leg. Expected workload differences were found between flying into high and low density airports and between long and short flights. First Officers' workload levels were significantly lower with a target-activated TCAS display. Nonsignificant trends in the Captains' and Second Officers' data also indicated lower workload for this TCAS format.