The conjugation of glycine to benzoates and the conjugation of L-glutamine to certain arylacetates are catalyzed by two different acyl-CoA:amino acid N-acyltransferases which can be purified separately from liver mitochondrial fractions of either rhesus monkey or man. In both species, one transferase is specific for glycine and the other for L-glutamine. The glycine enzyme utilizes either butyryl-CoA or benzoyl-CoA as acyl donors while the glutamine enzyme uses either phenylacetyl-CoA or indoleacetyl-CoA. Acyl-CoA substrates for one transferase do not serve as substrates for the other. Additional studies with the monkey liver enzymes revealed that acyl-CoA substrates for one transferase inhibit the other, that the apparent Km value is low (10(-6) to 10(-5) M range) for the preferred acyl-CoA substrate as compared to the amino acid acceptor (greater than 10(-2) M) and that both transferases have a molecular weight of approximately 24,000. Hippuric acid and either phenylacetylglutamine or indoleacetylglutamine were characterized as the products formed by the separate enzymes.