A dog, doubly depleted of blood cells and plasma proteins, was fed dl -lysine labeled with C 14 in the epsilon carbon position. In the first 8 hours 28 per cent of the administered C 14 was excreted in the urine; in the first 72 hours, 35 per cent. Twenty-four hours after feeding, 4.2 per cent of the fed C 14 was circulating in the plasma, decreasing to 1 per cent at the end of 17 days. The C 14 content of the blood cells increased from 1 per cent at 24 hours to 5.5 per cent in 5 days and 6.8 per cent in 22 days. Evidence based on the rate of decrease of the C 14 content of circulating blood cells is presented indicating an average life of 115 days for the erythrocyte protein as an entity not interchanging with extracellular constituents. This corresponds closely to the life span of the dog erythrocyte, 112 to 133 days according to the best evidence otherwise available and indicates that this experiment has actually measured the life span of the dog erythrocyte. Following breakdown of blood erythrocytes the protein comprising them is not used preferentially for the formation of new erythrocytes.