We evaluated critical variables in Bacillus Calmette-Guérin (BCG) immunotherapy of residual 13762A rat mammary adenocarcinoma. BCG was given intratumorally on Day 7 of tumor growth and followed by primary tumor excisions on Day 20. Untreated animals died on about Day 40 with axillary nodal and pulmonary parenchymal metastases. BCG-treated animals experienced prolonged survival, and some were cured. The highest dose (5.0 X 10(7) colony-forming units) of BCG was more effective than the lowest (0.5 X 10(7) colony-forming units), but 1,500 micrograms Corynebacterium parvum were more effective than even the highest BCG dose. Previous sensitization to BCG did not improve the effects of BCG treatment. BCG treatment was effective when given on Day 7 and sometimes as late as Day 12 or 17, but C. parvum was ineffective if given after Day 7. Repeated injections of BCG or C. parvum were not more effective than single injections were. Rats cured of residual 13762A tumor by BCG treatment were strongly and specifically immune to rechallenge. We concluded that a high dose (5.0 X 10(7) colony-forming units) of BCG given early (7 days) was the most effective presurgical treatment of 13762A metastases. Repeated injections or host presensitization to BCG did not improve the benefits.