The isotypic distribution of murine IgG was examined after infection with several viruses. The results indicate that when a hypergammaglobulinemia was induced by the infection, it was restricted to the IgG2a and, to a lesser extent, to the IgG2b subclasses. In addition, when mice were infected with some viruses concomitantly with the immunization with a soluble protein antigen, a modification in the isotypic distribution of antiprotein antibodies was observed, with a preferential production of IgG2a. These observations indicate that viral infections can actively influence the switch of Igs and selectively stimulate the production of the IgG2a subclass.