RECENTLY, two papers1,2 have appeared in which it has been shown that beryllium inhibits alkaline phosphatase by competition with magnesium. However, as inhibitions of more than 60 per cent were rarely attained, the results are of qualitative interest only. The authors were presumably aware of the insolubility of beryllium phosphate, since their alkaline phosphatase preparations were dialysed “to remove all traces of inorganic phosphate”2. But the inhibitor, beryllium, was added to the substrates (B. glycerophosphate or disodium phenyl phosphate) before the enzyme. These substrates undoubtedly contain inorganic phosphate in concentrations comparable to those of the inhibitor.