The immunogold method, utilizing 40 nm colloidal gold particles which can be selectively visualized with the scanning electron microscope (SEM) in the backscattered electron imaging mode was used for the study of blood cells incubated with various monoclonal antibodies. Numerous anti-leukocyte monoclonal antibodies still recognize lightly glutaraldehyde prefixed antigens and can be used to identify various blood cell types and even to recognize their different maturation stages. Clearcut differences in surface morphology exist among peripheral blood normal leukocytes and even among the principal lymphocyte subclasses. Marked heterogeneity in surface morphology is, on the other hand, evident when studying precursors or leukemic cells. Immature cells show, nevertheless, relatively smooth surfaces while some distinct surface features appear on cells already committed toward a specific differentiation lineage. Hairy cells can also be precisely identified, especially when in small number in heterogeneous populations, combining their typical surface morphology with their positivity for B1 and Leu M5 monoclonal antibodies.