Third-generation immunoassays for detection of antibody to human immunodeficiency virus (HIV) have reduced the interval between infection and antibody detection. Might such earlier detection diminish the value of the western blot as a confirmatory assay? We compared the sensitivity of one second-generation and two third-generation anti-HIV enzyme immunoassays (EIAs) with the results from HIV-antigen testing and western blot. 1045 western-blot confirmed anti-HIV positive samples were tested with a detection rate of 100% for all three EIAs. The detection rate in 36 samples drawn from different persons in early stages of HIV infection was 89% for the second-generation EIA and 94% for both third-generation EIAs. With carefully selected seroconversion panels that included sampling intervals during seroconversion of one week or less, we found that both third-generation EIAs detected seroconversion on average 5 days earlier (range 0-13) than did the second-generation assay. Western blot is commonly used to confirm HIV infection. In 6 of 10 seroconversions, one or both third-generation EIAs were reactive before any band appeared in the western blot. Since HIV antigen was detectable in these cases, the HIV antigen test may serve as a confirmatory assay for anti-HIV EIA-positive, western-blot negative, samples.