In 8 allergic asthmatic patients and 12 healthy volunteers, we investigated the effects of alveolar macrophages (AM) on lymphoproliferative responses to polyclonal T-cell mitogens of autologous peripheral blood mononuclear cells (PBMC). The AM were obtained by bronchoalveolar lavage. Peripheral blood mononuclear cells (PBMC) and mitomycin-treated autologous bronchoalveolar cells (BAC) were cocultured at various AM-to-PBMC ratios, and were stimulated or not by phytohemagglutinin and concanavalin A. Half of AM expressed HLA-DR antigens in both the asthmatic and the control group. The BAC from normal subjects were able to modify the lymphoproliferative responses of autologous PBMC to cell mitogens. A dose-dependent effect was observed related to the number of BAC added to PBMC--suppressive at high ratios but enhancing at low ratios. In allergic bronchial asthma, there was a decreased suppressor cell activity of BAC. Among BAC the adherent cells were responsible for this effect. This abnormality could be a part of more general decreased functional activity of AM in allergic bronchial asthma.