Abstract The rabbit reticulocyte is capable of utilizing purines and their ribosyl derivatives for nucleoside triphosphate formation. Adenine, adenosine, hypoxanthine, and inosine are used extensively for the synthesis of adenosine triphosphate. Xanthine, guanine, and guanosine are utilized primarily for the synthesis of guanosine triphosphate. The inhibitor 6-diazo-5-oxo-l-norleucine inhibits the utilization of xanthine for guanosine triphosphate formation in rabbit erythrocytes, but does not affect its utilization for the synthesis of adenosine triphosphate. Similarly, the inhibitor interferes with the utilization of xanthine for guanosine triphosphate formation in human erythrocytes, without affecting its utilization for the synthesis of inosine monophosphate.