Cyclophosphamide (CP) was administered to eight patients with metastatic bronchial carcinoma in escalating doses of 1.5, 2.5, and 3.5 g/m2 at intervals of 3 weeks. The proportion of the administered dose converted into alkylating metabolites was similar for each dose and there was no evidence to suggest that the enzyme system responsible for activating CP was saturated even with the highest dose. Considerable between-patient variation in drug metabolism was observed, but within each patient the fraction metabolised remained constant.