We investigated the influence of the macromolecular form of an epitope on the structure of antibody variable and constant regions expressed by the B cell population participating in an immune response to that epitope. Hybridomas were constructed from strain A/J mice undergoing either primary or secondary immune responses to p-azophenylarsonate conjugated to Brucella abortus (Ars-Bruc). We determined the sequences of the V genes expressed by hybridomas selected on the basis of expression of a single VH gene segment known to encode a large family of anti-Ars antibodies. These sequences were compared with the sequences of V genes expressed by a previously characterized panel of hybridomas isolated in the same way during the primary and secondary responses of A/J mice to Ars-KLH. The repertoire of Ars-specific V domains expressed among primary and secondary hybridomas elicited with these two forms of Ars were similar, as were the differences between primary and secondary V region somatic mutational alteration and affinity for Ars. In contrast, predominant expression of IgG2 anti-Ars antibodies was elicited in the secondary Ars-Bruc response, whereas secondary anti-Ars antibodies elicited with Ars-KLH are predominantly IgG1. Thus, differences in the macromolecular form of Ars clearly influence the isotypic profile of the anti-Ars response, but the expression, diversification, and selection of V domains elicited with this hapten are not greatly affected by such differences. Our results suggest that while isotype regulation is highly perceptive of the macromolecular form of a B cell epitope, V region regulation is primarily influenced by the molecular structure of that epitope.