The superoxide production of peripheral blood monocytes stimulated by concanavalin A and cytochalasin E was examined in sarcoidosis patients. The superoxide production of monocytes was significantly enhanced in sarcoidosis patients (9.8±6.4 nmol/min/10(6) cells) compared with healthy controls (6.6±3.5) (p<0.02). The levels of superoxide production were 12.4±8.7 in 14 patients with stage 1 sarcoidosis (bilateral hilar lymphadenopathy), 9.4±3.9 in 15 patients with stage 2 sarcoidosis (bilateral hilar lymphadenopathy and parenchymal involvement) and 6.1±2.6 in 4 patients with stage 3 sarcoidosis (parenchymal involvement). These levels were significantly higher in patients with stage 1 and 2 sarcoidosis than in healthy controls. The superoxide production was significantly enhanced in patients with elevated serum lysozyme activity and slightly enhanced in patients with high percentages of T-lymphocytes (over 28%) in bronchoalveolar lavage fluid. These data suggest that peripheral blood monocytes in sarcoidosis patients have been activated by unknown factors.