Guinea pigs injected with human leukemic substances exhibited a characteristic and severe disease, which presents the following clinical and histopathological picture: Marked reduction of red blood cells, loss of hair, skin atrophy, extensive degenerative changes of glandular parenchymas and histiocytic proliferation in spleen and lymph nodes. The etiological individuality of the disease and its relation to the injd. material is discussed. Thus, the disease is considered to be viral in etiology.