We treated 40 patients with superficial bladder cancer via intravesical bacillus Calmette-Guerin for 1) prophylaxis against tumor recurrence, 2) residual carcinoma or 3) flat carcinoma in situ. A single course of intravesical bacillus Calmette-Guerin therapy was successful in 6 of 11 patients (55 per cent) treated for residual carcinoma and 6 of 12 (50 per cent) treated for carcinoma in situ. Of 17 patients receiving a single course of bacillus Calmette-Guerin for prophylaxis 11 remained free of tumor during short-term followup. A second course of therapy was administered to failures in each treatment category, which resulted in favorable responses in 5 of 6 patients treated for prophylaxis, 2 of 5 treated for residual tumor and 3 of 6 treated for carcinoma in situ. Over-all complete responses were achieved in 16 of 17 patients (94 per cent) treated for prophylaxis, 8 of 11 (73 per cent) for residual carcinoma and 8 of 12 (66 per cent) for carcinoma in situ, with a mean followup from the final treatment of 9.3, 12.3 and 7.9 months, respectively. Favorable results occurred more frequently among patients who exhibited a granulomatous inflammatory response in the bladder and delayed hypersensitivity skin test response to purified protein derivative. Marked variability in viability of bacillus Calmette-Guerin organisms was observed among different lots of bacillus Calmette-Guerin, and a direct relationship was observed between bacillus Calmette-Guerin vaccine viability and therapeutic efficacy. Most patients who failed initial therapy with a low viability lot of bacillus Calmette-Guerin responded favorably to re-treatment with a higher viability lot. The results suggest that the level of viability of each lot of bacillus Calmette-Guerin vaccine should be verified before clinical use.