Significant differences have been found in the accumulation of glycine by avian erythrocytes and mammalian erythrocytes. In the former the glycine concentration is 3-5 times higher than that of the plasma , while in the latter it is approximately equal to that of the plasma ,,. While amino acid incorporation into avian erythrocytes is diminished by the presence of metabolic respiratory inhibitors such as cyanide, arsenate, and dinitrophenol, these agents do not affect the accumulation of glycine in mammalian erythrocytes . In the course of experiments on the relationship of amino acid transport and protein synthesis, we have studied the effect of insulin on the uptake of glycine by nucleated erythrocytes, which are capable of protein synthesis —as compared with nonnucleated erythrocytes, which are not capable of protein synthesis ,. We found that the nonnucleated cells accumulated glycine at a more rapid rate than did the nucleated cells, but the transport of this amino acid proved to be insensitive to insulin in both cell types. Freshly heparinized whole human or chicken blood was centrifuged and the plasma decanted. T'he cells were suspended five times in 510-100 volumes of 0.9% NaCl at 4°C. and subsequently centrifuged at 4°C. for 10 min at 465g. The incubation medium was prepared by mixing 2.15 ml of Raker's solution at pH 7.4 , 0.35 ml of Raker's solution containing 10 pc of glycine-14C and 10 mmoles of carrier glycine, and 0.35 ml of 10 U/ml insulin in a mixture of 3.3 mM hydrochloric acid in 0.9% NaCl. The temperature of the incubation medium was raised to 37°C. and 0.65 ml of packed erythrocytes were added. At zero time and at stated intervals thereafter, 0.5 ml aliquots were remosed and added to 0.5 ml of ice-cold Raker's medium. All further experiments were carried out in the cold. The cells were collected by centrifuging for 10 minutes at ~ 1050g.