Eighteen analogs of ATP have been tested in the aminoacylation reaction of phenylalanyl-tRNA and seryl-tRNA synthetases from baker's yeast. Four compounds are substrates for phenylalanyl-tRNA synthetase, five for seryl-tRNA synthetase, one compound is an inhibitor for both enzymes; their Km and Ki and V values have been determined. The substrate specificity shows that for the catalytic action of both enzymes with these substrates positions 6, 7, 8 and 9 of the purine moiety and positions 2' and 3' of the ribose moiety are important.