In order to further elucidate the mechanism of tRNA methylase-tRNA intreaction the methylation of some individual tRNAs separately and by pairs was performed. In conditions of tRNA excess the methylation rates of positionally analogous nucleotides in tRNA molecules are not summed up when two substrates are simultaneously present in the reaction mixture. The inhibitory action of yeast tRNASer, possessing m5c in position 29, on the methylation of C29 in other individual tRNAs was shown. Yeast tRNAVal which possesses an A residue in position 27 was shown to inhibit the methylation of G27 in E. coli tRNAMet. The data obtained confirm the suggestion that tRNA methylases recognizes the tertiary structure of tRNAs. They show also that the recognition and the proper catalytic action are two autonomous processes and that the former at least in its first stage is rather unspecific.