Cultures of peripheral blood mononuclear cells for human immunodeficiency virus type 1 (HIV-1) and assays for the p24 antigen were performed for a group of 75 unselected hemophiliacs to determine whether patients positive for HIV-1 antibody are actively infected rather than immunized by viral proteins in non— heat-treated factor VIII or IX concentrates. Fifty-six (75%) of the 75 hemophiliacs were antibody positive and 55 (98%) of the 56 with antibodies also had positive cultures. The one culture-negative individual had detectable HIV-1 proviral DNA sequences in three separate samples of peripheral blood mononuclear cell DNA, as detected by a polymerase chain reaction assay. Detection of serum p24 antigen and the time to development of a positive culture were significantly more frequent and shorter, respectively, in symptomatic vs asymptomatic patients. None of the 19 hemophiliacs negative for HIV-1 antibody had positive cultures, detectable p24 serum antigen, or symptoms of HIV-1 infection. Moreover, latent HIV-1 infection was not detected in 16 female sexual partners of hemophiliacs positive for HIV-1 antibody using Western blot testing, assays for p24 antigen, HIV-1 cultures, and polymerase chain reaction assays, despite repeated unprotected sexual exposure. We conclude that antibody-positive hemophiliacs have been actively infected by HIV-1 and that a long period of latent HIV-1 infection prior to overt seroconversion is unlikely.