Colony-stimulating factors (CSFs) stimulate granulocyte-macrophage praoduction from single hemopoietic progenitor cells. Various preparations of purified CSFs of two different subclasses have been shown here to stimulate a plasminogen-dependent fibrinolytic (plasminogen activator) activity from resident and starch-induced mouse peritoneal macrophages. Lymphocyte supernatants also stimulate macrophage plasminogen activator (PA) activity. Since they contain colony stimulating activity, it is possible that one or more subclasses of CSF in these supernatants is responsible for this effect. Since both colony-stimulating and macrophage growth activities have been detected at inflammatory sites, these findings could reflect a role for CSF in inflammatory processes.