The granulocytes in paroxysmal nocturnal hemoglobinuria (PNH) are defective; the defect is similar to that previously described for the PNH erythrocyte. The use of anti-I antibody to activate complement [C] and 51Cr release to detect cell lysis revealed 2 populations of granulocytes that differed in their susceptibility to lysis by C in 5 of 6 patients. A proportion of the cells were lysed by 1/15-1/20 the amount of C required to lyse normal cells. The remainder of the granulocytes appeared normal in their susceptibility to the lytic action of C. The binding of the 3rd component of C (C3) to PNH granulocytes was at least twice that bound to normal cells; the binding of antibody was the same for normal and PNH cells. The binding of C3 and probably the efficiency of the terminal steps of C lysis are increased in the abnormal PNH granulocyte. These defects affect only a portion of the granulocytes. The disorder is probably a clonal stem cell abnormality.