Biological impact of semen on virus infectivity and replication. Fifty samples of whole semen (Lee Biosolutions) were evaluated for biological activity in HeLa-CD4-LTR-β-galactosidase cells infected with varying quantities of infectious HIV-1_IIIB. Representative results obtained with six of these samples are presented. Semen was added to the cells in a volume of 50 μL immediately prior to the addition of infectious virus at 6 different virus inoculums ranging from a high inoculum of 50 μL (straight virus) and five additional serial twofold dilutions of virus in tissue culture medium. At 4 hours the virus and semen were washed from the monolayer of cells, and the cultures were incubated for an additional 48 hours at which time virus replication was quantified by β-galactosidase production in the cultures. The results presented demonstrate the three patterns of biological activity observed among the 50 tested samples: (1) enhanced levels of infection (see samples T3142 and T3125 at high virus inoculum), (2) inhibition of infection (T2073 and T3412 as well as T3125 and T3142 at lower viral inoculum), and (3) no effect on infection (T3553).