Surgical adjuvant intravesical bacille Calmette-Guerin (BCG) therapy is an effective method of treating superficial transitional cell carcinoma of the bladder. The role of the immune response in the antitumor activity of intravesical BCG is not known. We investigated the requirement of a thymus-dependent immune response for the inhibition of the growth of the intravesically implanted mouse bladder tumor, MBT-2. Intravesical BCG had no antitumor activity when administered to athymic nude mice bearing MBT-2 tumors. In two experiments tumor outgrowth in control and BCG-treated mice was identical. Adoptive transfer of BCG sensitized splenocytes (one spleen equivalent per mouse injected intravenously immediately prior to the first BCG treatment) syngeneic to the MBT-2 tumor transferred delayed hypersensitivity reactivity to BCG antigens and restored the antitumor activity of intravesical BCG. In two separate experiments mice receiving splenocytes plus BCG had 0 and 20% tumor outgrowth compared with 100% in control mice (p less than .02 and p less than .05, respectively). These results demonstrate that the antitumor activity of intravesical BCG therapy requires a thymus-dependent immune response.