C3b inactivator (C3bINA), a normal serum protein that destroys the ability of cells bearing C3b to participate in immune adherence, to undergo enhanced phagocytosis by polymorphonuclear leukocytes and to react with subsequent components of the complement system, was studied. C3bINA treatment of EAC14oxy23 cells prepared with purified and radiolabeled C3 released two bands of radioactive protein that were distinguishable from C3b on polyacrylamide disc gel electrophoresis. Similar products were demonstrable following the interaction of fluid phase C3b with C3bINA. The more anodal band appeared to be identical to C3c or β1A on the basis of electrophoretic mobility, behavior on gel filtration and antigenic content. The portion of C3 that remained on the cell following C3bINA treatment resembled C3d or α2D in its antigenicity. The action of C3bINA in vivo to produce cells coated only with C3d could account for absence of immune adherence reactivity and resistance to lysis of erythrocytes circulating in patients with certain forms of hemolytic anemia.