The murine interleukin 4 (IL4)-encoding cDNA expressed under the control of the immunoglobulin heavy chain enhancer/promoter was inserted into the mouse germ line. All B cells present in the IL4 transgenic mice show a marked increase in expression of class II histocompatibility (Ia) antigens compared to B cells from control mice. In vivo 5-bromo-2'-deoxyuridine incorporation and cell cycle analysis revealed no significant increase in B cell proliferation in spleens of IL4 transgenic mice compared to littermate controls. Likewise, serum immunoglobulin concentrations in IL4 transgenic mice were not increased. From these experiments we conclude that Ia hyperexpression on B cells per se does not induce activation of autoreactive T cells leading to B cell proliferation or hypergammaglobulinemia.