methodology of the colorimetric determination of alkylating cytostatics with 44nitrobenzylpyridine nbp in plasma the sensitivity of the colorimetric determination of alkylating cytostatics with 44nitrobenzylpyridine nbp has been increased by modifying procedures described in the literature this increase in sensitivity was accomplished as follows precipitation of protein with absolute ethanol at degree c reduction of the sample volume by evaporating the ethanol extract in a nitrogen flow control of the optimum ph value by means of a microglass electrode meticulously timed working under cooling in a dimmed room the total alkylating activity and the part of endogenously activated cyclophosphamide were determined in four patients