A model of tuberculous pleurisy in New Zealand white rabbits was developed to describe the sequential cellular and biochemical changes in pleural fluid. Bacille Calmette-Guérin (BCG) in 4 X 10(7) colony-forming units was introduced into the right pleural space of rabbits previously sensitized by intradermal BCG. Pleural fluid was obtained via serial thoracenteses. A normal-pH, normal-glucose, exudative effusion was seen through 144 hours. Polymorphonuclear leukocytes were the first cells to respond to the introduction of tubercle bacilli in the pleural space; they remained the predominant cell for the first 24 hours and were followed by macrophages, which peaked at 96 hours, and then by lymphocytes. Numerous granulomata were observed on both the visceral and parietal pleura ten days following intrapleural instillation of BCG. We propose that the polymorphonuclear leukocyte influx is not a nonspecific response to pleural injury and that such a leukocyte response, either itself or through its interaction with the macrophage, plays a role in host defense mechanisms against the tubercle bacilli.