The author, by the method of Weimer and Moshin, has determined the total serum polysaccharides (TSP) and the serum mucoprotein polysaccharide (SMP) in patients with gastric cancer, gastric ulcer, and in healthy persons. At the same time, sera in these persons were analized by the apparatus for paper electrophoresis, and stained with Amidoschwarz 10 B for serum protein, with the periodic acid Schiff's reaction (PAS) and the double oxidation toluidine blue reaction (DOT) for serum polysaccharides. The results obtained in patients with gastric cancer were as follows; 1) The TSP, SMP and those ratios to the serum protein were significantly elevated. Those levels were increased in parallel with size and metastatic spread of the tumor. 2) In the serum protein fractions, the decrease in albumin and increase in α(1),α(2) globulin were found. These changes were almost parallel to the growth and spread of the gastric cancer. No significant changes were recognized in β, γ globulins, but the latter increased at the late stage of the gastric cancer. 3) In the tests by the PAS reaction, the fall of albumin and elevation of α(1), α(2) globulin fractions were noticed. On the contrary, there were decrases in β, γ globulin. The elevations of α(1), α(2) globulin fractions were almost parallel to the growth and spread of the gastric cancer. 4) In the tests by the DOT α(1), α(2) globulin fractions were also noticed. There was an intimate relationship between the growth, spread of the gastric cancer and the fluctuation of α(1) globulin fraction. No gsinificant changes were recognized in the other fractions. 5) In patients with gastric cancer, the TSP, SMP, these ratios to the serum protein, and α(2) globulin fraction (DOT) were significantly increased more than those in patients with gastric ulcer. It would be said that the gastric cancer and the peptic ulcer were well differentiated in accordance with the level of SMP or the ratio to serum protein than the others.