The occurrence of immune complexes in the serum from rats infected with M. lepraemurium and 38 patients with leprosy were studied by the polyethylene glycol precipitation complement consumption (PEG-CC) test and the results were compared in the various forms of the disease. Circulating immune complexes (CIC) were significantly increased in the sera from rats infected with M. lepraemurium compared to normal control rats (P<0.005). There were no significant differences between the the level of CIC in the sera from lepromatous leprosy patients and that from tuberculoid leprosy patients, but in the sera from patients with erythema nodosum leprosum (ENL) the level of CIC was significantly increased (P<0.005). And although we couldn't find a corre1ation between the level of CIC and bacterial indices in lepromatous leprosy patients, CIC tends to de-crease after negative conversion of their bacterial indices.