The old and recent experimental contributions to the leukemia problem are discussed. Hereditary and hormonal factors, thymus influence and role of chemical and physical leukemogen agents are analyzed. The action of physical, chemical and hormonal therapies and of some viruses on the evolution of experimental leukemia is reported. Results obtained on a transplantable acute leukemia with some therapeutical trials are referred. The importance of Gross recent researches for the etiological problem of leukemia and tumours is emphasized.