A method is presented by which the site of primary attachment of the amino acids with respect to the 2'- or 3'-hydroxyl group of the terminal adenosine of E. coli tRNAs can be determined. It is found that the aminoacyl-tRNA synthetases (EC 6.1.1.-) with specificity for Arg, Asn, Ile, Leu, Met, Phe, Thr, Trp, and Val attach the amino acid to the 2'-position; those with specificity for Gly, His, Lys, and Ser attach the amino acid to the 3'-position; and that Tyr and Cys can be enzymatically attached to both the 2'- and 3'-positions. Together with previous experiments on yeast aminoacyl-tRNA synthetases, it is now shown that the specificity for one particular hydroxyl group is preserved during the evolution from prokaryotic to eukaryotic systems.