The optimal time interval between intralesional Bacillus Calmette-Guérin (BCG) and resection of s.c. tumor was studied in 70 female Fischer rats implanted with syngeneic 13762 mammary adenocarcinoma. If this tumor is resected about 17 days after implantation, all animals die from spontaneous pulmonary metastasis. Injection of BCG to tumor 3 to 7 days before resection, significantly improved the survival rates over rats that had resection of tumor only. About 22% of the rats so treated are long-term cures. The therapeutic effect of intralesional BCG was most pronounced in rats with smaller tumors, although BCG was effective in tumors over 2 cm. Intralesional BCG given 1 day prior to resection decreased survival for rats bearing larger tumors. The addition of postoperative systemic (s.c.) BCG immunotherapy did not improve the beneficial effect of preoperative intralesional BCG and resection of primary tumor.