Fe(59)-radioactivities in easily split off iron fraction (E S Fe(59)) of the rabbit erythrocytes labeled with Fe(59) were determined respectively under the each condition saturated with N(2) or CO gas and added with reducing agents such as Na(2)S(2)O(4) or l-ascorbic acid. The following results were obtained. 1. There was no significant difference in the yields of E S Fe(59) between the saturation with N(2) or CO gas and the exporsure to the air. 2. E S Fe(59) under the addition with l-ascorbic acid as reducing agents (E S Fe(59)(asc)) was decreased to 35.8% of the E S Fe(59) in the case exporsed to the air (E S Fe(59)(O(2)), while E S Fe(59) under the addition with Na(2) S(2) O(4) was rather increased. 3. The yield of E S Fe(59) under the addition of l-ascorbic acid in the presence of CO gas (E S Fe(59CO+asc)) was decreased to 23.1% of E S Fe(59)(O(2)). It was thought that the difference between these two yields indicate the artificial products as a result of hemoglobin oxidation. The inhibitory capacity in isolation of iron from hemoglobin was the largest in the presence of l-ascorbic acid and N(2) gas. 4. In the hourly determinations of E S Fe(59) in rabbit erythrocytes in vivo after intravenous administration of Fe(59), E S Fe(59)(CO+asc) was found to be the maximum in the immature cells, the minimum in the mature ones and tended to increase in older ones. These results were found to be same as that of E S Fe(59)(O(2)).