Studies were carried out on hamster bladder tumors with regard to virus detection and identification. Tumors were induced in hamsters by direct inoculation of Rh 911 strain of SV40 virus obtained from Flow Laboratories. Transformation of normal cells in tissue culture took about 21 days, whereas formation of tumors in hamsters took about 2-3 months. These tumors were studied under electron microscope as tumors and maintained in tissue culture. Electron microscopic studies revealed the presence of type C RNA tumor virus particles with all their budding stages. Also observed were H type virus-like particles with typical spikes. No virus particles could be detected in normal bladder. Further studies along these lines are in progress.