Thymidine phosphorylase (thymidine: orthophosphate deoxyribosyltransferase, E.C.: 2.4.2.4.) was detected by thin layer chromatography using PEI cellulose plates in human blood platelets. At a final concentration of 5.6 × 10−5 M thymidine the enzyme activity attained 1 µMol per hour and 1011 platelets. This enzyme was mostly found in mature cells, whilst its activity seems to be relatively low in immature and rapidly proliferating cells.