Ir-radiation of crystalline carboxypeptidase and of partly purified polyphenoloxidase by X-rays leads to the destruction of a constant amt. of enzyme by a given dose of X-rays independent of the conc. of the enzyme. In consequence the % destruction of the enzyme increases with decreasing conc. This is shown for a wide range of X-ray doses and conc. of the enzymes, very small doses of X-rays acting in the same way as big doses. When carboxypeptidase is irradiated while mixed with its substrate, no destruction occurs. Enzymes apparently play an important part in explaining the effect of radiation on living tissue. Their conc. and state during irradiation will determine the biological effect of radiation in those cases where they are a limiting factor in cell metabolism.