The protective immune response to human immunodeficiency virus type 1 (HIV-1) induced by vaccination will likely include cellular immune responses. We measured lymphoproliferative responses in persons vaccinated with a baculovirus-derived recombinant gp160 candidate AIDS vaccine. Twelve volunteers received either 40 micrograms of rgp160, 80 micrograms of rgp160, hepatitis B vaccine, or alum adjuvant alone on days 0, 30, and 180. Peripheral blood lymphocytes were collected on days 0, 28, 60, 120, 210, and 270 and were cryopreserved. Lymphocyte proliferation to mitogens and rgp160 with and without interleukin-2 stimulation were determined, and lymphokine production and antibody synthesis were measured. All vaccinees responded normally to stimulation with phytohemagglutinin and concanavalin A. One of 3 vaccinees who received 40 micrograms of rgp160, 2 of 2 vaccinees who received 80 micrograms of rgp160, and no controls developed rgp160-specific lymphoproliferative responses. No differences in the production of lymphokines (interleukin-2 and interferon-gamma) after stimulation with mitogens or rgp160 were found when rgp160 vaccinees and controls were compared. We conclude that rgp160 candidate vaccine induces antigen-specific lymphoproliferative responses in humans and does not interfere with immunocompetence as measured by in vitro responses to mitogen stimulation.