Procaine hydrochloride 0.1mg was added to the fresh serum 1ml of a surgical patient. After it was kept in an artificial incubator (37°C) for 30 minutes exactly, the amount of non-hydrolyzed procain in it was determined by the method described above. The 1 ml of normal human serum perfectly hydrolyzed 0.1 mg. of procaine hydrochloride in 30 minutes. On the other hand it was recognized that the activity of serum procaine-esterase decreased in some cases of stomach cancer, gastric ulcer, cholecystitis, splenomegaly and so forth. The amount of non-hydrolyzed procaine in these diseases was less than 0.03mg. But it is not always proper to say that the lowering of its activity in serum 1 ml suggests that of the total serum procaine-esterase activity of the same individual. On observing the relationship between hematocrit value and the activity of procaine esterase, the degree of anemia was more or less proportional to the activity of procaine-esterase in serum 1 ml. Furthermore investigating the total volume of circulating blood-plasma at the same time, we obtained the results as follows in the cases of gastric cancer patients, in which the activity of procaineesterase decreased markedly, the plasma volume often increased more than that in normal man. Therefore, in the view-point of the esterase activities of the total circulating plasma volumes, there was little difference between these patients and normal individuals. On the other hand, observing the relation between the amount of serum protein and the activity of procaine-esterase, it was suggested that the more the former decreased, the lower the latter often became, but the both were not always parallel, and also that the lowering of the latter might be related to the derease of albumin content.