Production of interferon (IFN) by Listeria monocytogenes (LM) in human peripheral blood mononuclear leukocyte (PBML) cultures was studied. Acid-labile IFN-gamma was produced in Ficoll-Hypaque-purified mononuclear cell cultures in response to heat-killed LM (HK-LM). However, plastic dish (PD)-nonadherent cells produced acid-stable IFN-alpha, whereas the cells reconstituted with PD-adherent cells (M phi) and PD-nonadherent cells produced IFN-gamma by HK-LM, IFN-gamma was produced only in a mixture of SRBC rosette-forming (T) cells and M phi. Neither T cells nor M phi produced IFN by HK-LM. On the other hand, IFN-alpha was produced from non-T and nylon wool-nonadherent (non-T, non-B) cells by stimulation with HK-LM in the absence of M phi. A reciprocal type of IFN produced in human PBML was absolutely dependent on the presence or absence of M phi. A soluble fraction extracted from sonically disrupted LM had a high potential to induce IFN-gamma in human PBML. Possible mechanism of the alternative production of IFN-gamma or IFN-alpha in human PBML by Listeria are discussed.