Ag-specific IgE and IgG antibody responses to a recombinant Ag-allergen chimeric (RAAC) protein were examined in B6D2F1 mice. The RAAC protein consisted of the truncated beta-galactosidase (beta-gal), linked at its C terminus to a polypeptide representing the conserved region of the recombinant Kentucky Bluegrass allergen encoded by the cDNA clone KBG8.3(rKBG8.3). Immunization of the mice with the RAAC protein in dextran sulfate as adjuvant led to the differential production of antibodies to the two constituents of RAAC protein with respect to their isotypic classes. Most of the antibody responses to both sets of determinants on the fusion protein were IgG1. In addition, the allergenic polypeptide of RAAC protein induced IgE antibodies, whereas the beta-gal elicited IgG2a antibodies. The same pattern of antibody isotypes was produced when the individual components, rKBG8.3 and beta-gal were separately used for immunization with dextran sulfate. On the other hand, immunization of mice with either RAAC or the beta-gal in CFA induced primarily a IgG response, and no IgE antibodies; however, under the same conditions of immunization rKBG8.3 induced IgG1 antibodies and also low levels of IgE antibodies. In contrast, the RAAC and the beta-gal induced IgG2a antibodies, whereas rKBG8.3 induced no detectable IgG2a antibodies. Furthermore, high titers of IgE antibodies were induced by the rKBG8.3 and not by the RAAC protein in dextran sulfate after the mice had been immunized twice with the same polypeptide in CFA. It is inferred from these results that the induction of isotype-specific immune responses in the animals with the same genetic background is dependent upon the Ag in question as well as the adjuvant; the latter, however, influences the magnitude but does not determine the isotype of the immune responses.