The author, with Shetlar's method, determined the serum polysaccharide of patients of cancer, sarcoma and tuberculosis; of patients of benign lesions; and of healthy persons; thus obtaining the following results. 1) In cases of cancer and sarcoma patients, the serum polysaccharide level is significantly elevated. The elevation is also found in case of tuberculous patients. 2) In 35 out of 43 cases (81.3%) of cancer and sarcoma patients, the ratio of nonglucosamine polysaccharide to serum total protein is not less than 2.0%. 3) The same ratio as above is found in 7 out of 40 cases of benign lesion patients and healthy persons. In them 1 case of Banti's disease, 3 cases of Graves' disease and 2 cases of struma are included. 4) This proves to be a diagnostic aid of cancer. 5) The serum polysaccharide level is also elevated by operation. 6) The surgical removal of cancer induces the fall of serum polysaccharide level, but this is hardly available in prognosis. 7) When the extract of cancerous tissue is injected into a rabbit, its serum polysaccharide level is elevated, and α-globulin also increaes parallelly.