T he red cells of patients with hereditary erythroblastic multinuclearity with a positive acidified serum test (HEMPAS), a form of congenital dyserythropoietic anemia, and the cells of patients with paroxysmal nocturnal hemoglobinuria (PNH) are lysed more readily than normal cells by certain antibodies, notably cold agglutinins (anti-I) and complement. With some but not other examples of anti-I, HEMPAS and PNH cells adsorbed more antibody than normal cells. Equal quantities of adsorbed antibody bound equal quantities of the first component of complement (C1) to normal, PNH, and HEMPAS cells. However, for a given quantity of bound antibody and C1, much more of the fourth component of complement (C4) was bound to HEMPAS cells than to normal cells. This resulted in the binding of proportionately larger quantities of the third component of complement (C3) to these cells. The same amount of bound C3 was found on the membranes of normal and HEMPAS cells for a given degree of lysis. Hence, the marked increase in lysis of HEMPAS cells is due to the increased adsorption of antibody and/or increased binding of C4.PNH cells bound the same amount of C4 per bound C1 as normal cells but bound more C3 than normal cells. However, the mean concentration of C3 on the membrane of PNH cells was one-third to one-fifth that on normal cells for a given degree of lysis. Hence, the increased lysis of PNH cells is due to the increased binding of C3 and increased hemolytic effectiveness of the bound C3.