In 8 patients receiving intravenous isophosphamide 2 g/m2 at 2-week intervals for advanced bronchogenic carcinoma the protective effect of 2-mercaptoethane sulphonate sodium (mesnum) against isophosphamide-induced urothelial toxicity was tested in a single-blind crossover trial. With isophosphamide alone, 7 of the 8 patients developed either haematuria or symptoms of bladder irritation; when mesnum was given in addition, only 1 patient had microhaematuria and frequency, and this was in association with a urinary-tract infection. 5 patients then received fifteen courses of isophosphamide in increasing doses of 4 to 8 g/m2 i.v. with mesnum. In contrast to previous experience with isophosphamide at this high dosage, frank haematuria was never seen, microhaematuria was seen after only three courses, and mild dysuria after only one course. Pharmacokinetic studies showed that mesnum did not interfere with the metabolism of isophosphoramide or its active anti-tumour metabolite, isophosphoramide mustard. Mesnum therefore enhances the therapeutic ratio of isophosphamide and may thereby increase its clinical efficacy.