The author studied the behaviors of bone marrow in acute bleeding rabbits by fluid culture of bone marrow and obtained the following. 1) Increasing rate of red cells immediately after bleeding is the same as that of the normal, but it begins to decrease 1 hour afterwards; and it shows the minimum value 8 hours later. However, it begins to rise 12 hours after bleeding; and its rate is 1.59 times that of the control 2 days after bleeding, 2.38 times after 3 days; 2. 55 times after 5 days; 1.98 times after 7 days; 1.50 times after 10 days. Namely, the muximum rate of increase in the erythrocytes is reached on around 3-5 days after bleeding and 14 days after it recovers more or less to the normal level. 2) Increasing rate of hemoglobin as compared with that in the control shows a decrease 1 hour after bleeding and it is lowest around 8-12 hours afterwards; but it begins to increase around 24 hours and shows the maximum value around 5-7 days after bleeding, while on the 14th day it practically coincides with that in the control group. However, the rate of production of hemoglobin seems to be less than that of red cells.