During immune hemolysis by human complement, C'3 (beta(1C)-globulin) becomes physically attached to the erythrocyte membrane. Binding of C'3 was found to be mediated by cell-bound, activated C'2 and to have the characteristics of an enzymatic reaction. A single C'4,2a site on the cell surface effected the binding of several hundred molecules of C'3 if the latter was provided in excess. The accumulation of hemolytically inactive, physicochemically altered C'3 in the fluid phase was found to be an inherent feature of the process of C'3 binding. It is postulated that C'4,2a activates C'3 for its subsequent reaction with cell membrane receptors. Antierythrocyte antibody did not play an essential role in C'3 uptake; C'3 could be bound to erythrocyte-C'4,2a complexes which were entirely devoid of antibody. Cell-bound C'3 proved hemolytically active, the degree of hemolysis being proportional to the number of C'3 molecules per cell. Binding of a large number of C'3 molecules per cell was found to be a prerequisite for the production of the immune adherence phenomenon and for immune hemolysis.