The optimum concentrations of leucine, ATP, GTP and Mg 2+ ion for the incorporation of leucine into protein by the microsomal fraction isolated from sterile disks of red beetroot are 0.06 mM, 5 mM, 0.5 mM, and 12 mM respectively. Incorporated 14C-leucine does not exchange with an excess of soluble-12Cleucine. Incorporation into protein is partly dependent on the addition of a high speed supernatant fraction which incorporates leucine into a product with the properties of aminoacyl I%NA. Addition of polyuridylic acid to microsomes isolated from fresh disks stimulates the incorporation of phenylalanine into protein ninefold but has no effect on leucine incorporation. Polyuridylic acid --stimulated incorporation is not inhibited by chloramphenicol. Preincubation of fresh microsomes with trypsin does not increase their activity. These results suggest that the low activity of fresh microsomes may be due to a lack of messenger RNA. The mitochondrial fraction shows a rise and fall in leucine-incorporating ability during aging similar to that shown by the microsomal fraction. Studies with inhibitors suggest that about 25 % of this incorporation is due to the mitoehondria themselves, the rest being attributable to large microsomes. Fractions isolated from disks aged under non-sterile conditions show large incorporations of leucine which are not dependent on an added energy source. This result confirms the importance of using aseptic techniques when studying the aging of storage tissue disks.