A case of hemocytoblastic leukemia in a 56 year old white man is reported which was characterized by cytoplasmic inclusions in some of the hemocytoblasts, reticulum cells and macrophages of the bone marrow as well as in some of the hemocytoblasts of the circulating blood. Phase and electron microscopic studies confirmed the corpuscular nature of the inclusions. Cytochemical stains gave negative results when tested for the presence of mucoproteins, lipids, phospholipids. neutral fat, desoxyribonucleic acid, peroxidase and phosphatases. The test for polysaccharides was slightly positive. Preliminary studies underway with electron microscopy suggest the possibility of a viral nature of the inclusion bodies.