The interaction between tryptophanyl-tRNA synthetase (EC 18.104.22.168) from beef pancreas and the ATP analogs containing alkylating or phosphorylating groups in the polyphosphate moiety of ATP was studied as an approach to investigate the structure of the enzyme active center. Some of the compounds under study were shown to irreversibly inhibit the enzyme activity; the presence of ATP in the most cases protects the enzyme against inactivation. The kinetic constants Ki and k2 of interaction of the irreversible inhibitors with the enzyme were determined. It was found that the Ki values for a number of irreversible competitive inhibitors are by 1-2 orders of magnitude less than the Km value for ATP; the k2 values were found equal to 0.02-0.04 min-1. this suggests that the compounds may be used as affinity reagents, the most efficient ones being adenosine 5'-(beta-chloroethyl phosphate) and mixed AMP-mesithylene carbonic acid anhydride. The absence of a protective effect of ATP in the case of adenosine 5'-(beta-bromoethane phosphonate) and non-competitive type of reversible inhibition inhibition of the enzyme by adenosine 5'-chloromethane phosphonate indicate that the molecule of tryptophanyl-tRNA synthetase contains sites interacting with adenine nucleotides, other than the ATP binding sites of the active center.