Morphological and cytochemical examinations of mononuclear phagocytes (MP) from venous blood leukoconcentrate were carried out in 25 patients with chronic monocytic leukemia, 7 patients with malignant histiocytosis, 3 patients with Langerhans' cell histiocytosis and 26 patients with reactive proliferations of the cells belonging to MP system associated with autoaggressive, infectious diseases or tumors. Morphofunctional features of MP from the patients with tumor and reactive histiocytosis may serve additional criteria in differential diagnosis of the diseases in which pathological process runs with participation of MP system cells.