Preparations of cell-free supernatant from Rous sarcoma "?????" were inocu-lated onto the chorio-allantoic membranes (CAM) of the chick embryos of 7--9 days in-cubation. The inoculated portions of the CAM were removed at various intervals from6 hours to 10 days afterwards. The tissues were fixed in Carnoy's solution and werestained with methyl green-pyronin for nucleic acid, Alcian blue-hematoxylin or Alcianblue-alum cochineal for acid polysaccharide and toluidine blue for metachromatic reac-tion. The pathogenesis of the Rous sarcoma of the inoculated CAM and the histo-chemical reaction of the nucleic acid and acid polysaccharide have been followed by his-tological examinations. Recognizable macroscopic lesions consisting of membrane opacity, focal thickening,oedema and discrete nodules in the inoculated CAM appeared from the first day on-ward. Distinctive hemorrhagic lesions and pocks were observed on the 3rd to the 5thday. Tumor masses of different sizes appeared from the 7th to the 9th day. In casethe CAM were inoculated with horse serum-saline, no or only slight reaction was ob-served, and the normal thickness and structures of the CAM were generally maintained. Three successive stages of the Rous sarcoma on the inoculated CAM weredistinguished: (1) Focal ecto-mesodermal proliferation with formation of earlytumor-like nodules; (2) Extreme ectodermal hyperplasia and early mesodermalsarcoma-like proliferation; (3) Ectodermal necrosis and extensive malignant growthof the mesodermal neoplastic cells. The fibroblasts, pyroninophilic round cells andthe ectodermal cells were the main types of cells concerned in the developmentof tumors following infection of the CAM Rous virus. The fibroblasts transformedinto tumor cells and became the most conspicuous elements of the Rous sarcoma. Thepyroninophilic round cells were considered to be belonging to the basophilic myelocyteseries, according to their morphological and staining characteristics. Their possible rolein the tumor infiltration was discussed. The intensity of the nucleic acid in the CAM cells increased along with the increaseof histopathological changes at the inoculated area of the membrane. The possible sig-nificance of their parallel relationship was discussed. Alcian blue staining of the growing tumor tissue demonstrated the presence of acidpolysaccharide reaction in the ground substance. The intensity increased with the increaseof the malignancy of the neoplastic cells. This material is non-metachromatic whenstained with toluidine blue and could be removed by treatment with testicular hyaluroni-dase for 3--5 hours. The mucin present is considered to be of a hyaluronic acid nature.