T-antigens of transplanted hamster tumors induced by human adenovirus type 12 were studied by fluorescent antibody technique within 24 hours after differential hypothermia (DH) treatment, in which one side cheek pouch tumor was warmed at 37±1℃ under generalized hypothermia of the body and the other side tumor at 20±1℃ for 10 hours. A high proportion of the tumor cells in the viable portions of the control tumors demonstrated specific fluorescent staining in three type. The most striking and consistent pattern was the presence of numerous fluorescent particles in the cytoplasm, which were granular and fleck-like shapes. A second type of fluorescence was fluorescent particles in the nucleus in addition to the cytoplasmic staining, and a third type was homogenous staining of nucleus. Immediately after DH treatment, the treated warmed tumor cells presented diminution of fluorescent staining, especially large numbers of granular fluorescences in the cytoplasms. Fleck-like and homogenous nuclear staining of the treated tumor cells scattered even at 10 and 15 hours after DH treatment, when treated tumor cells revealed necrobiotic findings in sections of HE and nucleic acid stain. But no fluorescence was observed at 20 and 24 hours after DH treatment, while not-treated hypothermic tumor cells demonstrated about same fluorescent staining as control tumor cells. It was suggested that cytoplasmic granular fluorescence diminished because DH treatment had an effect on thermo-sensitivity of T-antigen because DH treatment had an effect on thermo-sensitivity of T-antigen and/or inhibited T-antigen production, as well as loss of flecklike and hemogenous nuclear fluorescence was due to tumor cell necrosis by DH treatment.