Human serum contains resolved materials of protein and these resolution shows one section of protein metabolism. It is observed that the consumption of protein is clear in patients with cancer, and the quantity of non-heat precipitated protein-like materials increases in certain periods during the course of this disease. The examination of the resolving condition of protein was performed by means of the quantitative analysis of the tyrosine content in non-heat precipitated protein-like materials using the phenol reagent. In comparison with 3.2 mg% (2-5 mg%) which was the average tyrosine level of healthy human serum (10 cases), that of cancer of the lung (3 cases) was 11.2 mg% (8-17 mg%); of the stomach (42 cases) was 8.4 mg% (3-13 mg%); of the breast (8 cases) was 5.8 mg% (2-8 mg); of the rectum (5 cases) was 4.9 mg% (2-8 mg%); and of the sarcoma (5 cases) was 11.8 mg% (8-16 mg%). The tyrosine level of ulcer of the stomach and duodenum (35 cases) was 4.7 mg% (2-8 mg%), and this is lower than the level of cancer of the stomach. The tyrosine level of penetrating ulcer (5 cases), however, was 9.3 mg% (8-12 mg%) and it is higher than that of cancer of the stomach. In such diseases the resolution of protein is supposed to be evident, and this fact suggests hypoproteinemia and shows the necessity of supplying protein prior to operation in order to be favourable prognosis. Futhermore, the fact that the tyrosine level in the terminal stadium of cancer of the stomach with cancerous peritonitis becomes the same with that in healthy condition suggests us to be cautious in making diagnosis in the case of hypoproteinemia and cancer. Moreover, in the case of inflammatory and wasting disease except cancer. for instance, abscess of the lung, tuberculosis of the intestine, that of the kidney, and peripleural abscess, the tyrosine level becomes higher than 7-10 mg%.