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The effects of ionizing radiations on valyl-tRNA synthetase (L-valine tRNA ligase (AMP), E.C.6.1.1.9.), 100-fold purified from rat liver and Morris hepatoma 7777, are reported. The results showed that: 1) the enzymatic activity is strongly inhibited by radiations; 2) the enzymatic inactivation is much more marked in dilute than in concentrated solutions; 3) the inactivation of enzyme is linked to oxidation of sulphydryl groups indispensable to the catalytic activity of the enzyme; 4) no substantial difference in relation to inactivation by radiation was observed, between rat liver and Morris hepatoma 7777 enzymes.