After being analysed, upon experimental facts, the principal determinant causes of the normal cellular multiplication, and of the abnormal, found in the tumors, were studied the animal tumors caused by viruses like chicken sarcoma and epithelioma, rabbit, papiloma and infectious myxomatosis, rat and frog carcinoma and also oral papiloma and venereal lympho-sarcoma of dog. After words there was focused the various common properties existing between the induction viruses of such tumors like that of abnormal cellular multiplication in the attacked tissues, high specificity for the sensible species, reduction of their infectious power at same time enhancing the capacity of tumor formation and the existing affinities between them and the gens, specially those related to the X ray sensitiveness. Finally the A. formulated the hypothesis of a third pathogenic element between the cytocynetic virus and the gens, with a mild infectious power, acting only when special condition of susceptibility is present, which is not yet tottaly known nor controlled such element should be the responsible for the human neoplastic tumors.