Forty-one clinical patients were studied on the first day of the first course of the CMF-regimen; administered by a fixed dosage scheme depending solely on body surface area of the patient in question. Parmacokinetic parameters were calculated for each drug: the data were analysed in conformity with the usual pharmacokinetic models. The results indicate a large pharmacokinetic variability, especially for cyclophosphamide (C). The large variability in plasma concentrations of C is presumed to be substantially attributable to the drug formula used.