HIV can be recovered from infected patients at all stages of the disease spectrum. Typically, the quantity of biologically active virus, or viral protein, in body tissues is below the level of direct detection by either antigen capture or reverse transcriptase assays. Consequently, the virus must be expanded in culture. This may be achieved by the cocultivation of patient material with mitogen-stimulated peripheral blood mononuclear cells (PBMCs) from normal, healthy donors. These cocultures are then maintained by regularly scheduled interleukin-2 (IL-2) supplemented medium replacement, and the periodic addition of freshly stimulated normal donor PBMCs. During this cocultivation period, culture fluids are harvested at regular intervals and tested for the presence and subsequent replication of HIV. Cultures failing to demonstrate evidence of virus expression within 35 d are usually terminated.