1. The serum cholinesterase (ChE) activity in various blood disease generally shows a decreasing tendency, and it is marked in the acute form of leukemia. However, there can be recognized no special correlation between the serum ChE activity and the liver funktion and serum protein fractions. The serum ChE activity in blood diseases reflects well the clinical symptoms. 2. For the determination of the erythrocyte ChE activity in various blood diseases the author studied it by calculating the ChE activity value in a given unit of erythrocyte. As the result it has been found that in general the erythrocyte ChE activity in blood diseases shows an increasing tendency, and such a tendency is most marked in the acute form of leukemia. 3. When the administration of various drugs and blood transfusion prove effective on the fluctuations of the serum ChE activity and erythrocyte ChE activity in the course of treatment, these activities regain the normal level along with the recovery of anemia. The same trend can be recognized even in the experimental anemia.