C3Hf/Bi mice inoculated intracranially with human adenovirus type 12 were studied by fluorescent antibody technique and light microscopy.The cells with fluorescent T antigens were present in the wall of the ventricles and in the epithelium of the choroid plexuses. Tumors were induced in 21 of 45 mice and the predilection sites were the olfactory bulb, the floor of the fourth ventricle and the tapetum which was in contact with the hippocampus.The relationship between production of the T antigen and tumor development as well as histological features of the tumors was discussed.