Naturally occurring anti-band 3 antibodies appear to mediate opsonization of oxidatively stressed and in vivo aged red cells. Their low concentration in plasma (< 100 ng/ml) and weak affinity (estimated association constant, 5-7 x 10(6) l/mol) contrasted with their biological efficiency. In compensating for their inadequate properties they have an affinity for C3 at a site independent of the antigen binding domain, with an estimated association constant of 2-3 x 10(5) l/mol. Though weak, their binding to C3 was about 100 times higher than that of whole IgG, which is known to have an affinity for C3. The affinity for C3 may render these antibodies preferred targets of the short-lived nascent C3b and result in a preferential C3b-anti-band 3 complex formation. C3b-IgG complexes represent the best opsonins and can nucleate alternative complement pathway C3 convertases by which opsonization is further enhanced.