The effect of soluble factors from the monocyte/macrophage (M0) on cell proliferation and the functional effects of cell-cell contact on the arachidonic acid (AA) cascade were studied with vascular smooth muscle cells (SMCs). Peripheral blood M0s were isolated by adherence or in a Percoll gradient, and alveolar M0s were obtained by lavage. Conditioned medium (CM) was prepared by preincubating M0s with medium alone or by separating SMC and M0 cocultures by a membrane insert Cell proliferation (image analysis) and 6-ketoprostaglandin F1<r(6-keto-PGFla, radioimmunoassay) were measured in SMCs. Labeled prostanoids and other eicosanoid metabolites were isolated by high-performance liquid chromatography from SMCs prelabeled with14C-AA. M0s did not synthesize 6-keto-PGFIo. The CM enhanced proliferation but did not stimulate 6-keto-PGFi«, synthesis in SMCs. However, cell-cell contact in cocultures of SMCs with the same concentration of M0s used to generate CM resulted in increased 6-keto-PGFlasynthesis by SMCs. Since the stimulatory effect of cell contact was not blocked by butylated hydroxytoluene, it could not be attributed to an oxidative burst from M0s. Functional studies showed that the stimulatory effect of cell contact was enhanced by exogenous free AA and by endogenous AA release through A23187. Release of total radioactivity from prelabeled SMCs was enhanced by cell contact, and this effect was blocked by indomethacin (IM). Cell contact did not increase the release of free AA from prelabeled SMCs, even in the presence of IM. Finally, cell contact only stimulated the formation of prostanoids (IM-sensitive eicosanoid metabolites) from prelabeled SMCs. LJpoxygenase and other products of AA were not formed through cell-cell contact. These data showed that M0s express a soluble factor that enhances SMC proliferation without affecting prostanoid synthesis. Subsequent cell contact between SMCs and M0s stimulates prostanoid synthesis, which may possibly serve as a local and focal homeostatic mechanism for the regulation of uncontrolled SMC proliferation in atherogenesis.(Arteriosclerosis and Thrombosis1993;13:220-230) Conditioned medium (CM) was prepared by preincubating M0s with medium alone or by separating SMC and M0 cocultures by a membrane insert Cell proliferation (image analysis) and 6-ketoprostaglandin F1<r(6-keto-PGFla, radioimmunoassay) were measured in SMCs. Labeled prostanoids and other eicosanoid metabolites were isolated by high-performance liquid chromatography from SMCs prelabeled with14C-AA. M0s did not synthesize 6-keto-PGFIo. The CM enhanced proliferation but did not stimulate 6-keto-PGFi«, synthesis in SMCs. However, cell-cell contact in cocultures of SMCs with the same concentration of M0s used to generate CM resulted in increased 6-keto-PGFlasynthesis by SMCs. Since the stimulatory effect of cell contact was not blocked by butylated hydroxytoluene, it could not be attributed to an oxidative burst from M0s. Functional studies showed that the stimulatory effect of cell contact was enhanced by exogenous free AA and by endogenous AA release through A23187. Release of total radioactivity from prelabeled SMCs was enhanced by cell contact, and this effect was blocked by indomethacin (IM). Cell contact did not increase the release of free AA from prelabeled SMCs, even in the presence of IM. Finally, cell contact only stimulated the formation of prostanoids (IM-sensitive eicosanoid metabolites) from prelabeled SMCs. LJpoxygenase and other products of AA were not formed through cell-cell contact. These data showed that M0s express a soluble factor that enhances SMC proliferation without affecting prostanoid synthesis. Subsequent cell contact between SMCs and M0s stimulates prostanoid synthesis, which may possibly serve as a local and focal homeostatic mechanism for the regulation of uncontrolled SMC proliferation in atherogenesis.