Antibody to the binding site (idiotype) of anti-factor H was shown to have specificity for both B lymphocyte membrane H receptors and C3b. Goat F(ab')(2) anti-human H was purified by absorption and elution from H agarose and used for rabbit immunization to produce anti-anti-H (aaH). After absorption with nonimmune goat IgG, (125)I-labeled aaH bound to B lymphocytes and to sheep erythrocytes coated with C3b (EC3b) but did not bind to T lymphocytes or to EC3d. All B cell- and C3b-specific activities of the aaH were removed and subsequently recovered by absorption and elution of the antibody from either C3-agarose or goat-anti-H-agarose. This indicated that the aaH probably recognized a single common antigenic structure that was shared by anti-H, C3b, and the membranes of B cells. Affinity-purified aaH resembled H in that it bound to B cells, blocked the uptake of H onto B cell H receptors, and triggered B cells to release endogenous factor I (C3b inactivator). In addition, aaH functioned with factor I as either a cofactor for cleavage of fluid-phase C3b or a potentiator for cleavage of bound C3b. This same spectrum of C3 binding functions could not be demonstrated with either sheep anti-C3b or rabbit-anti-C3c. Analysis by sodium dodecyl sulfate- polyacrylamide get electrophoresis of the [(3)H]leucine intrinsically labeled B cell proteins reactive with the purified aaH revealed proteins of 100,000 M(r) and 50,000 M(r) without reduction, and after complete reduction of disulfide bonds, a single protein band of 50,000 M(r). This same protein molecular weight profile was also demonstrated with labeled B cell proteins that were absorbed and eluted from H-agarose. Thus, aaH is apparently specific for both B cell H receptors and C3b. However, because parallel analysis of C3b confirmed its known 115,000- and 75,000-M(r) polypeptide chain structure, the H receptor is probably not C3b and shares only the structure of the H binding site with C3b.