In the bone marrow of two patients with acute leukemia 46% and 55% of atypical eosinophilic cells were found, respectively. Blood eosinophilia was absent. The N-AS-D-Cl-Esterase reaction of the granules was positive in the first case in 58%, and in the second case in 3% of the eosinophils, as well as the PAS-reaction in all cells of this series. The ultrastructure of the eosinophils reveals nuclear maturation up to hypersegmentation. The maturation of the granules, in part of abnormal size, is arrested at the primary stage. Typical secondary granules with cristalloid cores are lacking. Only in the first case a few, small, semicircular or circular profiles of lamellar substructure are seen in the granules. The cytoplasm of hypersegmentated eosinophils shows an abnormally high glycogen content. Besides the eosinophils, monocytic cells and their precursors proliferate in the bone marrow of the first patient. In the second patient myeloblasts, promyelocytes with Auer rods, and monocytic cells characterize further neoplastic cell population. The elements of the monocytic series can be identified by their ultrastructural features, such as irregular configuration of the nuclei, bundles of cytoplasmatic microfilaments, and numerous small electron-dense lysosomal granules. In both cases the Alpha-Naphthyl-Acetase-Esterase reaction is weakly positive. The findings presented are summarized under the terms "eosinophil-monocytic leukemia" and "eosinophil-myelomonocytic leukemia" (collective term).