PUROMYCIN dihydrochloride is being extensively used as an inhibitor of protein synthesis. This antibiotic inhibitor is valuable because its mode of action on protein biosynthesis has been elucidated in precise molecular terms1. Examination of the literature shows, however, that there is a considerable range in the reported potencies of different samples of the inhibitor. For example, Wheelock found that 10−4 M (about 50 µg/ml.) puromycin completely inhibited protein synthesis in HeLa cells within 45 min of treatment2, while Shah's data, also on HeLa cells, show that twice that amount of puromycin (100 µg/ml.) allowed protein synthesis to take place at 20 per cent of the control rate after 6 h of puromycin action3. Numerous other examples of such discrepancies can be found4–7. This communication draws attention to the fact that different batches of the compound may vary in potency.