The protective effect of oral administration of the thiol compound sodium 2-mercaptoethane sulphonate (MESNA) against urothelial toxicity induced by ifosfamide (IF) was tested in a group of 45 patients with inoperable lung cancer under treatment with IF (2250 mg/m2 on days 2-5) as part of a polychemotherapy regimen repeated in a 4-week cycle. MESNA was given orally on the days of treatment with IF in 3 doses of 840 mg/m2, each administered at 0 hr (= injection of IF), 4 hr and 8 hr p.i. Out of a total of 88 courses of this treatment we observed 10 episodes of asymptomatic microscopic haematuria and no episodes of gross haematuria. In this group of 45 patients under protection with MESNA there were 5 complete remissions and 9 partial remissions (total 31%). A further group of 25 patients under polychemotherapy with IF were treated by conventional prophylactic measures (raised fluid intake and forced diuresis). In this group there were 1 complete and 5 partial remissions (total 24%), but nearly all patients developed either gross haematuria and/or symptoms of bladder irritation (cystitis and pollakisuria). There were no appreciable differences between the MESNA series and the conventional prophylaxis series with respect to either haematological or systemic toxicity of the cytostatic treatment. Our results support the view that MESNA, given orally in conjunction with combined cytostatic regimens which include IF, simplifies the treatment and provides optimum protection for the urinary epithelium. Protection with oral MESNA is particularly suitable for outpatients.