We used 3 techniques—freeze-etching, scanning, and thin-section electron microscopy—to study the ultrastructure of abnormal mononuclear leukocytes from 3 patients with leukemic reticuloendotheliosis. Freeze-etching minimized artifactual distortion of the cytoplasmic “hairs,” while allowing α three-dimensional-like view of the cell cleaved through membrane planes. Cleavage planes revealed no membrane abnormalities. Multishaped, tentacle-like projections of the cytoplasm contained few organelles. Cytoplasm contained mitochondria, endoplasmic reticulum, microfilaments, and granules. Nuclear pores were present in a high density similar to that in activated lymphocytes. Scanning electron microscopy, after critical point drying to decrease distortion, revealed abnormal cells with an extensive series of membrane outpouchings in these leukocytes but not in control preparations of cells from normal individuals and patients with chronic lymphocytic leukemia. Examination of thin sections revealed many cytoplasmic projections into redundant but apparently normal tri laminar membrane. Observation of these unique neoplastic cells by specialized ultrastructural techniques suggested that their major feature was the redundant membrane without unique morphologic membrane defects. The freezeetch studies confirmed that the hairlike projections of the leukocytes were not artifacts.