Semen and vaginal fluids transmit HIV infection. The virus is present as cell-free particles and as infected cells. Isolation of infectious virus from both genital tract fluids poses unique problems. Vaginal fluids are heavily contaminated with normal bacterial flora, and seminal plasma is cytotoxic to peripheral blood mononuclear cells. Adaptations of routine laboratory procedures have been developed to largely overcome these problems, allowing the culture and characterization of genital-tract HIV.