The detection and recruitment of HIV antigen-positive asymptomatic individuals for clinical trials is important. Two commercial enzyme-linked immunosorbent assays (ELISA) for the detection and quantitation of human immunodeficiency virus (HIV) antigens were evaluated for sensitivity by testing serum samples from 155 asymptomatic HIV Western blot positive individuals. The Abbott HIV antigen ELISA detected HIV antigen in the serum of 17 (11.0%) of 155 patients compared with 18 (11.6%) of 155 by the Coulter HIV antigen ELISA. In serial twofold dilution experiments, there was no significant difference in sensitivity between these two assays in the detection of HIV serum antigen. However, both assays are limited in their ability to detect HIV antigen in most asymptomatic HIV-infected patients. This low detection rate should be taken into account in the design of clinical trials involving asymptomatic infected patients.