The immune response of normal human peripheral blood mononuclear cells (PBMC) after stimulation with human immunodeficiency virus-1 (HIV-1) antigens plus Leishmania donovani promastigotes in vitro was investigated. HIV-1-antigen stimulation of PBMC did not induce the intracellular accumulation of interleukin-6 (IL-6), tumour necrosis factor-alpha (TNF-alpha), or interferon-gamma (IFN-gamma). However, cells stimulated with L. donovani antigens exhibited the production of IL-6 and TNF-alpha, but not IFN-gamma. Furthermore, co-stimulation of PBMC with HIV-1 antigen plus L. donovani resulted in the intracellular accumulation of IL-6 and TNF-alpha comparable to that of cells that were activated with L. donovani antigen alone. Heat-inactivated HIV-1 antigen did not appear to induce or suppress cytokine production by PBMC. However, the same HIV antigens did suppress L. donovani-induced proliferation as well as PPD-induced proliferation in a dose-dependent fashion. Elevated levels of serum cytokines have been demonstrated in patients with HIV infection indicating their role in the pathogenesis of HIV-associated immunosuppression. The results may partially support the idea that the abnormally increased cytokine levels in the sera of HIV-infected subjects is due to the various opportunistic pathogens that these patients contract, rather than a response to HIV antigens. As cytokines have been shown to up-regulate HIV replication, the data suggest a role for opportunistic infections in cytokine-induced transactivation of HIV-1 and disease progression.