The relationship between blood pH and blood coagulability and alternations in the fibrinolytic system was studied using blood samples obtained from the umbilical artery of 178 cases. Further studies were performed on this relationship in 5 cases of intracranial hemorrhage detected by ultrasonography, and the following results were obtained: 1. A definitely significant correlation was noted between the hepaplastin value and blood pH and these values markedly decreased in the acidosis group. 2. Changes in FDPs and SFMC were not so obvious as changes in the hepaplastin values, but activation of the fibrinolytic system was noted in the acidosis group. 3. No consistent finding was obtained with respect to changes in At-III. 4. Among five cases of intracranial hemorrhage, abnormalities in the coagulation and fibrinolytic system were noted in cases of acidosis, indicating that presence of acidosis is associated with severe intracranial hemorrhage. 5. The necessity of future studies on biochemical changes during acidosis was discussed.