Sera and ascites were gained from 72 cases of gastric canser and their protein fractions were estimated. The relationship between them and the grade of metastasis which settled during laparotomy was studied. 1) The protein fraction of serum in gastric cancer patients treated in surgery clinic showed no significant difference from that of normal ones. Albumin decreased its density and ratio as metastasis proceeded. γ- and β-globulin showed an increasing tendency with the proceeding of metastasis. The former increased markedly with the proceeding of peritoneal metastasis. The γ/A ratio showed the proceeding of gastric cancer most strikely. 2) The protein fractions of ascites increased and decreased in proportion to that of serum. Albumin stood always about 10% higher than serum. γ-globulin increased with the proceeding of peritoneal metasis, while γ-globulin decreased if the metastasis advanced. In the ascites the A/G ratio never reversed. 3) The most permeable fraction was albumin and it descended in the order of γ-, β- and γ- globulin. In the advanced peritoneal metastasis, and γ-globulin were very permeable. 4) Cases in which liver metastasis was found showed high protein, low albumin and high γ-globulin. In ascites it was also similar. 5) In patient with gastric cancer, peritoneal metastasis played the leading part in fluctuation of protein fractions. There was no significant difference between lymphnodes and infilatration on the serosa of stomach and protein fractions.