One hundred seventy-seven persons submitted specimens for serum neutralizing antibody titer detreminations 30 to 90 days after starting postexposure rabies prophylaxis. Ninety-two percent of those who received duck embryo vaccine alone developed adequate antibody titers. However, 23% of those who received equine antirabies serum plus duck embryo vaccine failed to develop an adequate antibody titer; one of these inadequate responders subsequently died of rabies. Factors that increased the risk of a poor antibody response included the receipt of steroids during the course of postexposure prophylaxis, and the use of more than 55 lu/kg of equine antirabies serum. Many persons receiving postexposure rabies prophylaxis fail to develop adequate humoral immunity and may have an increased risk of developing rabies. We suggest that persons receiving postexposure rabies prophylaxis should have serum neutralizing antibody determinations 30 to 40 days after starting treatment.