The osmotic and mechanical red cell fragility of the professional blood donors, who were found to be anemic as the result of frequent and repeated blood loss the past 5-6 years, were compared with that of the normal person while incubating the blood at 4-6℃ for 28 days. The fragility was expressed as % hemolysis occured during the incubation, and the following results were obtained: 1. The osmotic fragility in the normal persons (i.e, ; control group) progressively increased as the incubation became longer, and % hemolysis in 0.42% NaCl solution at 0, 10, 15, 21 and 28 incubation day was 31.90, 50.20, 41.68, 43.50 and 55.40 respectively. The mechanical fragility. in the normal red cells ranged between the minimum of 0.00% to the maximum of 5.80% both in 0.90 and 0.66% of NaCl solutions. 2. The hemolysis curve obtained in the red cell osmotic fragility from three cases of the anemic persons (i.e,; experimental group) showed a significant left side shift comparing with the normal in general which indicates that the fragility was more increased in the experimental group. The mechanical fragility in the experimental group ranged between the minimum 0.00% to the maximum 19.00% both in 0.90 and 0.66% of NaCl solutions. 3. The red cells of the chronic anemic person due to the frequent blood loss as the professional blood donor exhibit significantly marked increase both in osmotic and mechanical fragility comparing with the normal, and the tendency was more prominent as the incubation period became longer.