The effect of infection with living Bacillus Calmette-Guérin (BCG) on tumor growth was evaluated. Complete tumor inhibition was observed when infection with living BCG occurred at the site of tumor inoculation. This inhibition of tumor growth was not attributable to a direct cytotoxic effect of living BCG on tumor cells. Inhibition of tumor growth was mediated by the host during a delayed hypersensitivity-type immunologic response to the infecting organisms. Animals that rejected tumor cells at the site of an infection with BCG showed suppressed tumor growth and delayed hypersensitivity reactions to a subsequent challenge of tumor cells. The minimum number of living BCG capable of eliciting a host response that would completely kill tumor cells was 6 × 106 organisms in immunized and 6 × 105 organisms in unimmunized animals. Optimal suppression of tumor required living BCG, close contact between tumor and BCG, and a cell-mediated immune response by the host.