Hemopoietic cells display a wide range of cell surface antigens which are either lineage specific or acquired during differentiation. Monoclonal antibodies can be used, in conjunction with colloidal gold markers, to identify under the scanning electron microscopy (SEM) at the single cell level, specific lineage or maturation stages in the hemopoietic bone marrow. Normal bone marrow cells, either gradient separated or purified by immuno-magnetic methods and leukemic cell samples, which can be considered as "frozen" stages of hemopoietic differentiation, have been studied with this method. Typical cell surface morphologies, which characterize immature progenitor cells and cells committed or differentiated towards the lymphoid, myeloid, erythroid and megakaryocytic lineage have been identified. Correlations between cell surface features and some hemopoietic cells functions have been attempted on the basis of these findings.