A case is reported of a 15-year-old boy with prolidase deficiency and marked urinary excretion of the iminodipeptide gly-pro. Prolidase activity of erythrocytes against substrate glycyl-proline was deficient, but after blood transfusions this was increased to 15.7% of donor activity and declined to 12% and 3.4% of normal activity after 8 and 45 days, respectively. Urinary iminodipeptide levels following transfusion remained unaltered.