During the last ten years since Yoshida discovered his ascites tumor (1943), many authors have intended to isolate a certain living agent immediately from the tumor ascites, but nobody succeeded. In 1950 Y. Hamazaki et al. have theoretically concluded that the tumor cell must contain a certain virus, because they have verified many fine granules of polymerized desoxyribonucleic acid in the cytoplasm which were demonstrated by Feulgen's reaction. And then, about a year later we and I fortunately succeeded in isolating and successive transmissions of a new virus (virus HST) from the tumor ascites and proofed sero-immunologically that the virus is nessesary to complete the transplantation of the ascites tumor successfully. Method of isolation. Mice which were transmitted intracranially with 0, 02 cc of the ascites, as a rule, died in a range of 5-6 days under serious brain symptoms with paralysis. The agonized animals were killed to prepare an emulsion of their brain which was diluted decimal with bouillon and centrifuged 2500 r. p. m. for 5 mins. By several successive intracraianlly transmissions of 0, 02 cc of supernatant from brain emulsion, the animals became very sensitive and paralysis disappeared. Transmitting it continuously (on the course of passage, filtration of the emulsion by Seitz's filter-plate was done once a time), we could obtain a fixed virus causes a certain meningoencephalitis in mice. The other case, we have immediately transmitted the ascites afresh into the abdominalcavities of normal mice. 4-6 days later, they suffered from the heavy ascites and gradually became very sensitive too. Their brains were made into emulsion in the same way as mentioned above and intracranial transnsmissions of it was done successively. Histological findings, 1) Mice inoculated with the ascites. Dissecting the cranial cavities you can find an unclean, yellow, thick foil on the pia mater. The tumor cells multiply limitlessly in and out of the brain. The meningitis takes place chiefly in the ventral surface of the brain and the dorsal surface of the midbrain. The glia cells, especially the subependymal fibrillary glia cells often multiply in enormous numbers and infiltrate near by. 2) Mice inoculated with the isolating virus. Similar histological changes, as above mentioned, could in miniature be seen. Especially, changes of the adjacent area of the lateral ventricle cavities and the edema of the brain parenchyma or the hydrocephalus interna often could be seen characteristically.