We describe an assay procedure for the quantification of pyrimidine nucleoside phosphorylase, the enzyme thought to degrade the pro-drug 5'-deoxy-5-fluorouridine to 5-fluorouracil. The method is based on the known differences in the ultraviolet absorption spectra in alkaline medium between pyrimidines and their nucleosides. Analogous to the cleavage of uridine by uridine phosphorylase, the enzymatic degradation of 5'-deoxy-5-fluorouridine in the presence of inorganic phosphate yields 5-fluorouracil and ribose-1-phosphate. We have shown that the rate of phosphorolysis of this pyrimidine nucleoside could be readily measured by spectrophotometry. The method described is simple, specific for the 'pro-drug' as well as reproducible. We have also applied this method to define tissue enzyme activities in extracts of human tumours, normal tissues of the same organ and tissues of mice.