Recent studies have shown that in vitro exposure of peripheral blood mononuclear cells (PBMC) to morphine results in suppressed respiratory-burst activity of monocytes and impaired interferon-gamma (IFN-gamma) production by lymphocytes. To investigate the potential in vivo effect of an opiate on these cell functions, PBMC were obtained from patients maintained on methadone. These freshly isolated mononuclear cells had a significantly impaired capacity to generate superoxide anion (O2-) in response to phorbol myristate acetate (PMA), while production of IFN-gamma by concanavalin A-stimulated cells was intact. After cell culture for 48 h, the defective O2- generating capacity was sustained. Also, culturing PBMC from healthy controls in the presence of methadone or morphine at concentrations as low as 10(-12) M caused significant suppression of PMA-stimulated O2- release. Because reactive oxygen intermediates produced by PBMC may participate in host defense against opportunistic pathogens in AIDS, these results underscore the need for investigations of the biological consequences of opiate-mediated immunosuppression.