The ability of active specific immunotherapy to enhance immune responses to autologous tumor-associated antigens (TAA) and to prolong the disease-free interval was evaluated in patients with Dukes B2 and C colorectal carcinoma who had undergone potentially curative resections. Patients were sensitized in the early postoperative period with irradiated autologous adenocarcinoma cells mixed with bacillus Calmette-Gu6rin (BCG) to yield either a low-dose vaccine (3 Γ 106 tumor cells) or a high-dose vaccine (1 Γ 107 tumor cells). Six of seven patients who received the low-dose vaccine developed delayed-type hypersensitivity (DTH) responses to autologous tumor cells upon completion of the vaccination, whereas all four patients receiving high-dose vaccine displayed a positive DTH response. However, DTH responses to autologous TAA waned within 3 months in all patients receiving the low-dose vaccine; DTH responses persisted for 3 months in three of the four high-dose vaccine patients. In vitro lymphoproliferative responses to TAA correlated with DTH responses to autologous tumor cells. Active specific immunotherapy appeared to induce specific immune responses either in vitro or in vivo to autologous TAA because it did not induce responses to autologous mucosa cells. There were no complications caused by BCG or tumor cells. This series demonstrates that active specific immunotherapy is a nontoxic treatment that augments immunity to autologous TAA.