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Chemical modification of lysine residues of eukaryotic tyrosyl-tRNA synthetase was studied. It was shown that only four out of 22 lysine residues per enzyme dimer could be modified with pyridoxal-5'-phosphate. This modification led to the inactivation of tRNATyr aminoacylation by more than 90% but did not practically affect the rate of ATP-[32P]pyrophosphate exchange. Low molecular weight substrates (ATP, ATP-tyrosine) weakly protected the enzyme from inactivation, whereas tRNATyr afforded a much more effective protection. It was supposed that lysine residues of tyrosyl-tRNA synthetase can be involved in the interaction with tRNATyr.