Murine antibody responses to soluble proteins are generally restricted to the immunoglobulin G1 (IgG1) isotype. When mice were infected with Toxoplasma gondii Beverley and concomitantly immunized with a soluble unrelated protein antigen, a modification in the isotypic distribution of antibodies directed against this nonparasite antigen was observed, with a preferential production of IgG2a. Interestingly, when mice were immunized with a soluble protein antigen during the chronic phase (day 40) of infection with T. gondii Beverley, a similar modification in the isotypic distribution of antiprotein antibodies was observed.