Mechanisms of immune response associated with immunotherapy remain controversial, and the role of the antibody response in the efficacy of immunotherapy has largely been ignored. By the use of immunohistochemistry to search for the presence of antibodies and antibody-secreting plasma cells, the role of antibody response immunotherapy was examined. By the use of an antimouse immunoglobulin antibody reactive against all isotypes, cells consistent with plasma cell morphology and immunophenotype were observed more frequently in the tumor (but not normal brain) of vaccinated mice. Additionally, high levels of what appeared to be non-specific and diffuse localization of secondary antibody also were observed. This staining is absent, however, in B/plasma-cell-deficient mice, suggesting that secreted antibodies are present in the tumors of wild-type mice. Animals deficient for B/plasma cells did not respond to immunotherapy when compared with vaccinated wild-type animals. Thus, the use of immunohistochemistry has guided the experimental design and clarified mechanisms of immune system response to immunotherapy, demonstrating that tumor-infiltrating plasma cells or circulating antibodies account for the endogenous reactivity to antibodies observed in brain tumors.