The injection of thioglycollate medium into the peritoneal cavity of the mouse induces high levels of macrophage fibrinolytic activity due to the production and secretion of a plasminogen activator, a trypsinlike serine protease, which is absent in unstimulated macrophages. Intraperitoneal injection of endotoxin or mineral oil can stimulate only a fraction (<10%) of the fibrinolytic activity of thioglycollate cells, similar to the partial stimulation (<10%) seen 1-2 days after phagocytosis of latex or SRBC by unstimulated macrophages. The endotoxin-stimulated macrophages contain and release relatively low levels of plasminogen activator, but these primed cells can be triggered to produce and secrete high levels of enzyme, by phagocytosis of latex. Under conditions where the plasminogen activator is induced and secreted, there are no effects on the production and/or release of lysozyme or intracellular acid hydrolases, Discovery of a two-stage procedure for inducing macrophage plasminogen activator made it possible to study the role of cell priming and phagocytosis separately. Endotoxin was a more effective priming agent, weight for weight, than lipid A:BSA complex. Secretion of the plasminogen activator was induced only by thioglycollate, or endotoxin and latex. In situ fibrinolysis was induced by these agents and mineral oil, BCG, and fetal calf serum, in decreasing order of effectiveness. Phagocytosis of latex in all cases except thioglycollate stimulation, increased fibrinolytic activity from three- to sixfold. Latex and a variety of other particles such as M. lysodeikticus, aggregated gamma-globulin and immune complexes showed dose-dependent stimulation of fibrinolysis by endotoxin-primed macrophages. Although the initial phagocytic trigger was not specific for the substance employed, the ability to induce a sustained response depended on the persistence of the phagocytized particle within the cell. Fibrinolysis and secretion of plasminogen activator continued at high levels for at least 9 days after uptake of latex, a nondigestible particle, whereas plasminogen activator was secreted only transiently after ingestion of rapidly digested M. lysodeikticus. The induction of plasminogen activator secretion provides a mechanism by which the activated macrophage can exert a selective effect on its extracellular environment.