Circulating immune complexes were investigated in sera of 13 patients with paroxysmal nocturnal hemoglobinuria. A significant inhibition was observed in the complement-dependent rosette formation (EAC) in 6 cases; 2 patients presented an increased anticomplementary activity; antilymphocyte antibodies were demonstrated in 2 patients. The levels of C3d and immunoconglutinins were also increased; there were no differences in the total hemolytic complement between patients and normal controls. There was a significant correlation between C3d and immunoconglutinins with the inhibition of the EAC rosette formation test. These results provide evidence for the presence of circulating immune complexes in paroxysmal nocturnal hemoglobinuria.