Although human immunodeficiency virus (HIV) type 1 infection is efficiently transmitted by sexual intercourse, some individuals whose sexual behavior places them at extremely high risk for infection have nevertheless remained HIV-1-seronegative. An investigation was undertaken to determine whether such individuals have circulating T helper cells that are sensitized to HIV-1. Five very high risk men who had recent sexual exposure to HIV-1 were studied. Peripheral blood mononuclear cells from all 5 produced interleukin (IL)-2 in culture in response to synthetic amphipathic HIV-1 envelope peptides. One of the 5 high-risk men has subsequently seroconverted, while 4 have remained seronegative. All were initially culture-negative, and those who have remained seronegative were also virus-negative by polymerase chain reaction (PCR) testing 10 months after they were first studied. These results demonstrate that a cell-mediated immune response to HIV-1 can be detected in the absence of a humoral immune response in individuals recently exposed to HIV-1. Furthermore, IL-2 production by T cells in response to synthetic peptides may be a more sensitive test for exposure to HIV-1 than antibody, lymphoproliferation, or PCR tests.