We have shown that the labile binding site of C3b interacts covalently with receptive surfaces. We report here an analogous study of the interaction between the labile binding sites of the closely related complement proteins, C4 and C5, with sheep erythrocyte membranes. We find that i) C4b binds covalently to cell surface components; ii) the bond between C4b and receptive molecules is hydroxylamine sensitive; iii) the alpha-polypeptide of C4b binds to receptive molecules; and iv) C5b does not interact covalently with cell surfaces.