Getting a hint from immunologic specificity of bacterial polysaccharide, the author, with Julianelle's method modified by Hosoya, isolated the polysaccharide-like substance from cellabundant and actively proliferous cancerous tissue, and applying it as antigen to patient serum in the precipitation reaction, got the following good results in diagnosis of cancer and other malignant tumors. 1) Obtained diagnosis rates are as follows: the positive results in case of patients with cancer, sarcoma and other malignant tumors ……88%; the negative results in case of patients of benign lesions ……85.1%; the average hitting rate in total cases ……86.5%. However, the differential diagnosis between cancer and sarcoma is impossible. 2) Cancerous tissue applied as antigen, when it is abundant in cells and increasing actively, shows better precipitation reaction (71.7-97.1%). Precipitation reaction by the antigen isolated from a ripe pracenta shows the result of 71.8%. While, the antigen isolated from non-cancerous tissue shows no reaction. 3) Substance of antigen consists mostly of polysaccharide, very small quantity of protein or protein-like substance being mixed in. 4) The antigen isolated from cancerous tissue by an improved method of resolution with bile, shows a successful result. 5) The application of serum-diluting method makes better result, but this does not agree with the results of complement fixation reaction and skin test. Though the true character of this precipitation reaction is unknown at present, it seems to be something like an immunologic specific reaction. 6) Above mentioned reaction is found to be pretty much available in early diagnosis, while it is of no avail in prognostic judgment.