Intravesical bacille Calmette-Guérin (BCG) has been shown in prospective randomized clinical trials to be the treatment of choice for superficial bladder cancer. In this investigation we evaluated the role of CD4 and CD8 lymphocytes in the antitumor response. Monoclonal antibodies to thy 1.2, CD8, CD4 and an isotype control were injected intravenously to deplete T cell populations. After depletion (verified by flow cytometry), BCG therapy was initiated. The results demonstrate that the depletion of either CD4 or CD8 T cell subsets eliminated BCG-mediated antitumor activity. Footpad delayed type hypersensitivity (DTH) was aborted only in CD4 depleted mice; it was essentially unchanged in CD8 depleted mice. However, the presence of DTH was not sufficient for induction of BCG-mediated antitumor activity. Exogenous IL-2 at levels sufficient to induce lymphokine activated killer cell activity did not substitute for CD4 cells. There was no evidence for the induction of protective immunity to the tumor after BCG therapy. These results demonstrate the requirement for T lymphocytes in BCG-mediated antitumor activity and further demonstrate that the presence of both CD4 and CD8 subsets are required. CD8 depletion experiments suggest that the presence of CD4-mediated DTH is not sufficient for the induction of antitumor activity. Furthermore, these data suggest that BCG-mediated antitumor activity is a localized phenomenon that does not induce protective immunity.