To evaluate the effector functions of human lung macrophages, cell preparations containing 70 to 95% macrophages were obtained from surgically resected lungs of cancer patients and were co-cultured with allogeneic or autologous peripheral blood mononuclear cells (PBMC) and Con A. In contrast to previous reports of either marked stimulatory or inhibitory effects of human lung macrophages on lymphocyte function, the present results demonstrate that the proliferative response was a complex function of both the numbers of PBMC and macrophages. In the presence of low numbers of PBMC, small numbers of macrophages enhanced proliferation, whereas larger numbers inhibited proliferation; in the presence of larger numbers of PBMC, macrophages only inhibited. Inhibition was not mediated by cyclo-oxygenase products of arachidonic acid metabolism, because indomethacin did not reverse it. The enhancing effect of macrophages was greater when tested with PBMC depleted of monocytes. Lung macrophages were 10-fold more potent in mediating enhancement than corresponding numbers of peripheral blood monocytes. Both the enhancing and the inhibitory activities could be reproduced by lung macrophage lysates or supernatants derived from macrophages cultured in serum-free medium. Macrophages cultured at high density yielded inhibitory supernatants, which on dilution resulted in enhancing activity. The enhancing activity appeared in supernatants maximally after 24 hr, and its appearance was not inhibited by culturing macrophages in the presence by culturing macrophages in the presence of cycloheximide. Sephacryl S-200 chromatography of such supernatants yielded two peaks of enhancing activity, with apparent m.w. of 160,000 and 40,000, which we call lung macrophage-derived lymphocyte-activating factors (LM-LAF). Fractions with LM-LAF activity contained no IL 1 activity (assessed by augmentation of mitogen-induced proliferation of mouse thymocytes), but IL 1 activity was present in a peak of m.w. of 15,000. The 15,000 m.w. fraction did not enhance the proliferation of human PBMC. These results demonstrate that human lung macrophages are potent modulators of lectin-mediated proliferation of human PBMC. The effects are mediated in part by the release of soluble, pre-formed factors that appear to be distinct from previously described monokines.