High levels of plasminogen activator (PA) were induced in mouse peritoneal macrophages by infection with BCG, 2-6 X 10(7) viable organisms intravenously, followed 3-4 wk later by intraperitoneal challenge with purified protein derivative (PPD) 2 days before harvest. Macrophages obtained from infected animal without boosting showed little fibrinolytic activity, but challenge of Bacille-Calmette-Guèrin (BCG)-primed peritoneal cells with PPD in culture also enhanced macrophage PA 4- to 10-fold. Stimulation of macrophage PA by PPD depended on specifically sensitized thymus-derived (T) lymphocytes because it was abolished by pretreatment of BCG-primed peritoneal cells with anti-thy 1.2 antiserum and complement. A direct assay was developed in which nylon wool separated sensitized lymphocytes and PPD induced PA in macrophages from uninfected animals under defined conditions on 125I-fibrin. Enhanced macrophage fibrinolysis was proportional to concentration of PPD and the number of sensitized lymphocytes transferred. An indirect two-stage assay was also used to show that BCG-sensitized peritoneal cells released a soluble inducer of macrophage PA into the culture medium, after challenge with PPD. Induction of macrophage PA by PPD challenge in vitro made it possible to study the generation and activity of sensitized peritoneal lymphocytes at different stages of infection. Our results show that nonadherent peritoneal cells of BCG-infected mice provide a rich source of specifically sensitized lymphocytes and that macrophage activation is limited by continued availability of antigen, as well as sensitized lymphocytes. Induction of macrophage PA provides a sensitive, versatile, and rapid in vitro assay to study the role of lymphocytes and specific antigen in macrophage activation by BCG.