A direct colorimetric assay of ammonia liberated after a guanase reaction with 8-azaguanine as substrate was tested using a guanase-assay kit (Maruho Co., Ltd.). The clinical importance of the guanase activity in blood was studied. The method for determination of guanase activity in the blood with this kit was simple, sensitive and accurate indicating that this kit could be useful as a routine test. The level of guanase activity of 775 normal subjects was 0-3.8 IU/L. A good correlation between guanase and transaminase in the blood was obtained in patients with hepatitis. In normal subjects there was not a good correlation between the two enzymes. To prevent the development of hepatitis after a blood transfusion, it was found that the determination of guanase activity in blood was more useful than the screening of GOT and GPT activities. Sixty-one per cent of the subjects developed hepatitis after blood transfusion. In a group which received blood containing less than 2.0 IU/L of guanase activity, less than 16.7% developed hepatitis. For a group which was transfused with blood containing more than 2.1 IU/L of guanase activity, 80% of the patients developed hepatitis. These results indicate that the determination of guanase activity is a useful test to determine the probability of transmitting hepatitis. To avoid the transmission of hepatitis during a blood transfusion, it is necessary to use blood containing low guanase activity.