The use of mitomycin C, an effective agent in the intravesical treatment of superficial bladder cancer, is limited by its high cost. An in vitro study was done to determine whether mitomycin C could be recovered after intravesical administration and reused in the same patient. Cultured human transitional carcinoma cells from line 253-J were exposed to: medium alone; fresh mitomycin C (1 mg./ml.); mitomycin C stored for 1, 2 and 4 weeks; and mitomycin C recovered from patients after intravesical use, tested immediately and stored and tested after serial dilution after 1, 2, and 3 weeks. Results were assessed by cell counts 2 and 5 days after 2-hour exposure to the test solutions. With fresh mitomycin C and that stored up to 4 weeks, cell growth was less than 1 per cent of control (medium alone) at 2 and 5 days. Mitomycin C was also effective when recovered after intravesical use for up to 2 weeks, at which time cell growth was 9.7 and 3.1 per cent of control at 2 and 5 days, respectively. However, at 3 weeks, cell counts were 11.5 and 18.3 per cent of control at 2 and 5 days, suggesting that mitomycin C loses potency with dilution. We conclude that mitomycin C might be recovered, stored and reused for at least 2 weeks in individual patients, a practice that could result in a substantial reduction of treatment cost.