The tendency was observed that the esterified vitamin in the blood decreased after blood transfusion in 16 cases. In general, the decrease of esterified riboflavin in the blood was observed after transfusion of preserved or fresh blood with or without the administration of riboflavin. When mixed blood (29 groups) in a ratio of 1 : 1 was incubated in vitro at 37℃ for 24 hours, the contents of total riboflavin, its esterified form and their ratio were markedly decreased as compared with that of non mixed blood (the control) at a significant level. A small increase was observed in the esterified riboflavin when ATP was added to mixed blood and the mixture was incubated at 37℃ for 24 hours. These results indicate that the decrease of esterified riboflavin in the blood after transfusion seems to be due to mixing up of the blood, and especially chiefly due to the decrease of organic phosphates such as ATP. Consequently, the simultaneous use of ATP was rational in transfusion of the blood for preventing from the decrease of esterified riboflavin in the blood after the transfusion.