Receptor recognition of pertussis toxin is mediated by the B oligomer consisting of subunits S2, S3, 2xS4, and S5. One possible way to interfere with toxin action would be the inhibition of recognition and binding of the cellular receptor(s) by preformed toxin-directed antipeptide antibodies. A prerequisite for this approach is the localization of linear antigenic determinants followed by the identification of inhibitory epitopes. Anti-S2 peptide antibodies have been shown to inhibit binding of the holotoxin to in vitro model receptor systems. For the elucidation of linear antigenic and immunogenic determinants harbored in the S3 subunit, synthetic peptides corresponding to selected linear amino acid sequences of S3 have been prepared and used to raise peptide-specific antibodies in rabbits. All peptides elicited a strong homologous response. Four synthetic peptides reacting with anti-pertussis toxin antibodies (R36-51, R87-95, R134-150, and R147-160) have been identified. Seven synthetic peptides (R1-12, R12-23, R14-29m, R36-51, R95-107, R134-150, and R164-178) induced antibodies recognizing pertussis toxin. Thus, these segments correspond to linear antigenic determinants. Analogous to the S2 subunit, the N terminus of S3 proved to be immunorecessive in the native toxin. The highly homologous S2 subunit was only bound strongly in Western blotting (immunoblotting) by antiserum directed at peptide R164-178, which is identical in the S2 and S3 subunits. A weak recognition of S2 in Western blotting was observed with anti-R95-107 antiserum. The ability of affinity-purified anti-S3 peptide antibodies to interfere with pertussis toxin binding was investigated by hemagglutination of goose erythrocytes as a model receptor system for S3-mediated receptor recognition. Antipeptide antibodies directed at R1-12, R12-23, R14-29m, and R36-51 inhibited hemagglutination of goose erythrocytes. This indicates that the corresponding antigenic regions in the S3 subunit are associated with the formation of the receptor binding domain. Inhibition of B-oligomer-mediated pertussis toxin binding to cellular receptors by preformed antipeptide antibodies of sufficient affinity should not only block the detrimental effects of the S1 subunits, but also interfere with the mitogenic effects attributed to the B oligomer.