With the purpose of elucidating the role of the stroma for the growth of tumors, the vascular structures in the parenchymal stroma and the surroundings of animal tumors were studied. In the first chapter, the vascular structures of Bashford tumors and Ehrlich subcutaneous tumors were examined employing the India ink method. The Blood vessels of tumors are derived from their surrounding host connective tissue. The structure of blood vessels of every tumor has features of its own. Further, it was observed that metastatic tumors in the lungs of Ehrlich ascites carcinoma and Brown-Pearce tumors had an individuality in their vascular structures. In the second chapter, the vascular structures of Ehrlich subcutaneous tumors and first generation transplants of C(3)H spontaneous mammary cancers were observed employing ultrasoft X-ray, and the relationship between the blood vessels and the stromaltissue of these tumors was investigated using several special staining methods. Characteristic features were also observed on the vascularization and the stromal tissues of Ehrlich subcutaneous tumors treated with chondroitin sulphate. In the third chapter, the influences of administration of Bleomycin on C(3)H spontaneous mammary cancers were abserved with respect to the vascular struture and stromal tissue. It was found that small tumor vessels were narrowed, obstructed and destroyed by administration of this drug. Further, it was observed that the vasculature and stromal tissue of lungs of mice and humans administered with this agent were likewise damaged, and that prednisolone and chloroquine considerably inhibited these pulmonary damages caused by this drug.