The cytostatic activity of five Bacillus Calmette-Guérin (BCG) strains (Pasteur, Evans, Tice, RIVM and Connaught) on human transitional cell cancer T24 cells was examined. A striking effect was noted even in 2-day cultures, and the effect was more pronounced when the cells were incubated for 5 days with different BCG strains alone. The concentrations needed were about the same as those used in clinical practice (10(9) colony-forming units of Pasteur strain in 100 ml buffered saline solution). Combination with mitomycin C or interferon-alpha-2b potentiated the cytostatic effect. A slight difference in cytostatic activity between different BCG strains was found.