Human blood, drawn into ACD, was stored under blood bank conditions with and without addition of tritium-labeled progesterone. At selected storage intervals, several biochemical and biophysical parameters were assayed and the distribution of the hormone between plasma and erythrocytes was determined. The retention of the radioactive material by erythrocytes was studied by repeated washes and isotonic saline. The results indicate that erythrocytes stored with addition of progesterone have higher ATP levels, less spontaneous lysis, higher osmotic resistance, and higher uptake of methylene blue throughout the time of storage than do cells stored without added progesterone. This hormone appears to be in steady-state equilibrium between plasma and red blood cells during 42 days of storage and the amount in the plasma is almost twice that present in the cells. After repeated saline washes, about 10 er cent of the progesterone remains attached to the erythrocytes. Specific association of progesterone with red blood cell populations of different density shows that high specific activity is obtained in cell populations of low density (young cells) throughout the storage time investigated.