1. Transferase I of rat liver binds aminoacyl-tRNA to form a relatively stable complex, which is retained on cellulose nitrate filters. This reaction proceeds at both 0°C and 37°C and is inhibited by GTP. The resulting product is stabilized by GTP and Mg2+. 2. Only very low quantities of deacylated tRNA are bound by transferase I. 3. Methods are described for the preparative isolation of the transferase I–aminoacyl-tRNA complex from incubation mixtures by using ion-exchange procedures. 4. The transferase I–aminoacyl-tRNA complex becomes readily bound to ribosomes. The presence of Mg2+ is essential for the binding. GTP stimulates this reaction but is not absolutely required. 5. It is concluded that the formation of the transferase I–aminoacyl-tRNA complex may be the primary reaction in the binding of aminoacyl-tRNA to mammalian ribosomes and that, unlike in bacterial systems, GTP is not absolutely required for this step.

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