Investigation with 57Co-Bleomycin in patients with the various cancers and in tumor bearing animals are descirbed. In the patients, 57Co-Bleomycin appears to be one of the useful tumor- seeking radiopharmaceuticals, and worth applicable to clinical uses. Labelled yield of 57Co-Bleo was about 97% by thin layer chromatography. The pyrogen free tests were performed to meet U.S.P. critical ranges. In clinical studies with 57Co-Bleo, 4 cases out of 5 patients with lung cancer., 2 cases among 3 thyroid cancer patients, and all 3 hepatoma patients showed positive tumor scans. The patients with stomach cancer, and the esophageal cancer showed false negative scintigraphy. A case with pulmonary tuberculosis showed a positive scan while liver abscess showed a negative picture. The merits of 57Co-Bleomycin scintigraphy seems to be its relatively high affinity to tumors and low radiation hazard in spite of long physical half life.