By means of immunofluorescent technic, the author investigated the sequential changes of the viral antigens and tumor antigens in the tissue of newborn hamsters inoculated with adenovirus tyqe 12. 1. Subcutaneous inoculation Viral antigens could not already be found 4-5 hours after virus injection, but tumor antigens diffusely appeared in the injected part at least 24 hours later. In process of time, tumor antigens were gradually localized to the corium and muscle layer. Atypical large cells were recognized in the intermuscular bundles 15 days after inoculation, stained with strong, brilliant fluorescence. Histomorphologically they were identified with tumor cells originated from immature nerve-supporting cells which corresponded to muscle spindles or motor endplates. 2, Intraperitoneal inoculation Viral antigens could not almost be found 4-5 hours after injection, followed by appearance of tumor antigens 12-24 hours later. On the observation of the mesenterium tumor antigens also gradually localized and became more and more strongly stained on the nerve fibers, spindle shaped cells (probably schwann's cells) as well as ganglion cells. In 7-10 days after inoculation, small tumor nodules which showed bright fluorescence were found closely related with nerve fibers. On the other hand, the tumor antigens on the serosa cells became gradually weak and the degeneration of serosa cells was demonstrated after 3-7 days, which resembled cytopathic effect of infected HeLa-cells in vitro. 3. Intracranial inoculation Viral antigens disappeared more rapidly (one hour after virus inoculation) and tumor antigens were gradually located principally in the ependymal layer, meninx, but sometimes in the parenchyma of the brain which related to small blood vessels. Moreover, viral antigens were detected after three hours in the Kupffer cells of the liver and sinusendothelial cells of the spleen.