Intraperitoneal injection of the serum of colon cancer carrying WF rats induced, within two months, colon carcinomas in the ascending colon of LE and Wistar/Shi rats when they were given it during their suckling. We had also induced colon carcinomas in the ascending colon of ACI rats by the same methods. Therefore, it is supported that this serum derived from colon cancer carrying WF rats must have some transmissible agent in itself. In addition, we ultracentrifuged the serum of cancer carrying WF rats and we found, in the sediment, numerous round or oval virus like corpuscles by electron microscopy studies. Negatively stained corpuscles by phosphotungstic acid staining clearly revealed fine spike appearance on their surface. We believe that these virus like corpuscles are the etiological agent for the transmissible colon carcinoma of WF rat strain.