The myelo-monocytic leukemias (MML) can be of acute, subacute or chronic type, according to the level of bone-marrow blast cells and to the spontaneous course. We have compared the clinical, hematological and biological characters of 11 cases of subacute MML-defined by a survival of less than 12 months - to 20 cases of chronic MML. Anemia, hyperleucocytosis, monocytosis, number of circulating granulocytes and blast cells, level of bone marrow blasts, were superior in acute MML, with significant difference for hemoglobin, circulating and bone-marrow blasts. A high urine lysozyme output, a decrease of granulo-monocytic colonies after bone-marrow culture on semi-solid media were further arguments in favor of the subacute type. This variety, which evolves into acute MML in near 50 p. cent of cases appears consequently as the first step of an acute leukemia, or represents a very close aspect. Chronic MML on the contrary, although with still 30 p. cent blastic transformation, is characterized by a steady course, with a median survival of 3.3 years, and number of untreated patients surviving many years beyond this median time.