By turbidity method, the hyaluronidase activity of the tissue extract and the antihyaluronidase activity of the serum were compared between patients with gastric cancer and gastric ulcer and the following results were obtained: 1. HD-activity of the carcinomatous tissue was much higher than that of binign ulcerous tissue, and anti-HD-activity of the serum in cancer patients was also more increased than that in the patients of gastric ulcer. 2. In general, activities of HD and anti-HD showed the tendency to increase or decrease in parallel with extracellular fluid (Thiocyanate space). 3. HD-activity of the microscopically normal stomach tissue distant from the carcinomatous ulcer was decreased remarkably than that of the carcinomatous tissue. 4. On the carcinomatous ulcer itself. it was recognized that HD activity is little different according to the part or place of the ulcer. 5. The remarkable differences of HD-activity among Borrmann's types were not recognized, but type III and IV showed a little higher activity than that of type I and II. 6. As the result of experiments on dogs, it was confirmed that hyaluronidase increased extracellular fluid and the same action was recognized in the cancerous extract also. 7. In the experimental study on rabbits, it was recognized that hyaluronidase accelerated the diffussion's ability of the pigment in the intracutaneous tissue.