Direct gene transfer into muscle can lead to sustained gene expression at the injected sites. Here we tested the ability of in vivo gene transfection to immunize mice against an isolated human IgV region. The Humkv325 germline kappa L chain V gene encodes the kappa L chain in V regions of several human IgM autoantibodies and is used frequently in chronic lymphocytic leukemia. This gene was inserted into a mammalian expression vector, pREP7, to produce pREVk3. Mice injected i.m. with pREVk3 produced antibodies against the V region of Glo, a human monoclonal IgM paraprotein whose kappa L chain is encoded by Humkv325. Co-injection of an expression vector encoding IL-2 enhanced anti-Glo antibody production fivefold and induced a localized delayed hypersensitivity reaction. Antibody production also was induced when vectors encoding Humkv325 and IL-2 were injected s.c. These experiments demonstrate that gene immunization vectors can stimulate immune responses to antibody V region determinants.