A commercial enzyme immunoassay was utilized for the detection of the human immunodeficiency virus antigen (p24) in serum samples from persons at risk for AIDS. The test demonstrated sensitivity to 20 pg/ml of serum antigen. The results obtained showed 14.63% of the subjects (Ss) with persistent generalized lymphadenopathy or asymptomatic Ss, and 69.23% of the Ss with AIDS related complex (ARC) or AIDS to be positive for HIV-Ag. In addition, there were no false-positives in the control serum samples collected from seronegative individual. The close correlation observed between HIV-Ag presence and advanced stages of the disease, stresses the diagnostic importance of this viral marker and its usefulness in the follow-up of Ss with HIV infection.