We sterilely homogenized the tumorous tissue of 3 choriocarcinomas confirmed by histological examination and made them emulsion by adding five times volumes of normal saline. After then it was centrifuged 3.000 r. p. m. for 5 minutes and each of these supernate were inoculated to 2 healthy guinea pigs which then were killed 2 weeks later by blood-loss. We autopsized them to take out sterilely their brain which then were made into 10% emulsion by adding normal saline. 0.2cc of these emulsion was inoculated to healthy guinea pigs which we called 2 nd generation. We successively incculated with same procedure. And also these guinea pigs were microscopically examined by H-E staining on their brains, lungs, livers and spleens. Regarding 1st case and 3rd case on the time of inoculation from fourth to fifth generation, and 2nd case frcm 5th to 6th generation the passage through Seitz E. K. filter was performed. The inoculation was continuously performed up to 22nd for 3rd case. The histological changes in the organs of guinea pigs were mainely found as the hyperplasia of septal cells and hypertrophies of septum, resulting in frequent narrowing or disappearing of alveolar cavities. There were present the infiltrations of lymphocytes and monocytes in septum. Around the vessels and bronchioles the hyperplasia of mesenchymal cells was revealed. The pathological findings shown above were indentical with interstitial pneumonia what we call virus pneumonia. There were present the hyperplasia of spider cells and infiltrations of monocytes and lymphocytes around the bile duct as liver concerned. Spleen showed the hyperplasia of reticulocytes in red marrow. All three cases were revealed to have histological changes even after the passage through Seitz filter and mede it possible to continue the successive inoculation. There was, however, found in 1st and 3rd cases the tendency of gradual withdrawal of histological changes. 2nd case, contrary to this, kept the almost same pictures after having passed through the filter. Using the brains of guinea pig proper for isolation of tumor filtrable agents and avoiding the infection from experimental room, we successfully isolated the filtrable agent from choriocarcinoma, and believe that this agent is choriotropic.