It has been shown previously ( von der Decken and Hultin, 1958, Hultin and von der Decken, 1959) that in, vivo -labeled sRNA ∗ ∗ PEP, phosphoenol pyruvate; TCA, trichloroacetic acid; sRNA, soluble or transfer RNA; 32P-sRNA, sRNA labeled in, vivo with 32P-phosphate. prepared from rat liver is transferred to the ribonucleoprotein particles of rat liver microsomes. Similar results have also been obtained by Hoagland (1958) and Bosch, Bloemendal and Sluyser (1959). The sRNA molecules are specific for individual amino acids and current evidence strongly suggests that the various amino acids are transferred by sRNA to the ribosomes. The question of how the ribosomes direct the alignment of amino acids in a specific sequence has been clarified recently. Thus, messenger RNA has the ability to attach itself to the ribosomes and there to determine the sequence of amino acids (Risebrough, Tissières and Watson, 1962). Very little is known, however, about a possible interaction between sRNA and messenger RNA when the amino acids are lined up on the ribosomal template during the process of peptidization. One link in this complex system, the enzymic relationship between the transfer of sRNA and of amino acids from sRNA to ribosomal particles, will be described in this paper.