The conception is still general that but two types of leukemia exist: myelogenous, arising from the bone-marrow, and lymphatic, arising from the lymphoid tissue. That there is a third form of leukemia, arising from the reticulo-endothelial (histiocytic) system and characterized hematologically by an extreme degree of monocytosis and histiocytosis, is not as yet well recognized. Comparison of the reported cases indicates their similarity and emphasizes the existence of a clearcut entity, monocytic leukemia. Reschad and Schilling-Torgau,1 in 1913, were the first to describe this "new" form of leukemia. Since then twenty-six cases have been reported. The most complete studies have been made by Merklen and Wolf.2 The present communication describes the observations in two additional cases. REPORT OF CASES3 Case 1.—History. —M. R., a Jewish business man, aged 55, was admitted to the hospital on Oct. 21, 1928, complaining of swollen gums. The family and marital history were not significant.