Reappraisal measurements of apparent half survival time of red cell by $^{51}Cr$ method was made and effects of blood-letting over red cell survival were observed. The study was performed on 53 normal male subjects under three different experimental conditions. 1. Group 1 Mean $^{51}Cr$ red cell half survival by ACD wash method was 29.7 days. $Tfrac{1}{2}$ of Ascorbic acid method was 29.0 days in group with 100 mg dose and 29.1 days in group with 50 mg dose respectively. There was no difference between these two methods in regards to red cell half survival. No difference were noted in amount of ascorbic acid administered. 2. Group 2 As daily amount of blood loss is increased the shortening of red cell half survival was noted. Rapid phase was seen when blood loss ranged 10 to 25 ml per day, while slow phase noted when more loss amounted 25 ml or more daily. Thus, it was clear that there was more than an exponential relation between $Tfrac{1}{2}$ and the amount of blood loss. 3. Group 3 $Tfrac{1}{2}$ measured by cpm per whole blood was within normal range and $Tfrac{1}{2}$ measured by cpm per red cell mass showed shortening tendency when compared with the former in the group measured after blood loss (from 25 ml daily up to 100 ml daily in 10 days). In the group with rather constant blood loss of 100 ml daily for 10 consecutive days revealed the significant difference in two measurements (P<0.01). 4. $Tfrac{1}{2}$ in non-steady state When red cell production is increased compared with red cell destruction, $Tfrac{1}{2}$ measured by cpm per red cell mass being shorter than that by cpm per whole blood. Shortening of $Tfrac{1}{2}$ measured by cpm per whole blood is more prominent. if red cell destrction is enhanced and exceeds production. 5. It is clear that when expressing red cell destruction rate, $Tfrac{1}{2}$ measured by cpm per whole blood is more adequate and production more consistent with cpm red cell mass. 6. $Tfrac{1}{2}$ measured during blood-letting, when corrected by amount of blood loss, it remains normal. It is erroneous to use conventional equational when measuring $Tfrac{1}{2}$ in non-steady. $Tfrac{1}{2}$ measured by cpm per whole blood is considred more applicable in clinical evaluation.