Macrophages obtained from the peritoneal cavity of untreated mice do not ordinarily synthesize plasminogen activator. However, induction of enzyme synthesis and secretion occurs when such macrophages are cultured in presence of conditioned medium from Con A-stimulated spleen cells. Plasminogen activator production by macrophages from endotoxin or thioglycollate medium-injected mice, which spontaneously secrete substantial amounts of the enzyme, is also markedly increased in presence of such conditioned medium. These results suggest that macrophage plasminogen activatory production may be regulated in part by lymphocytes. They provide further evidence to link macrophage plasminogen activator with cell migration and inflammation, and also support the view that in macrophages, as in certain other cell types, synthesis and secretion of this enzyme are under hormonal control.