Résumé : We analyzed mononuclear leukocytes from patients with various human leukemias by high-resolution two-dimensional electrophoresis. Tumor cells of the granulocytic, monocytic, and lymphoid lineages [obtained from chronic granulocytic leukemia in blast transformation, acute monocytic leukemia, and chronic lymphocytic leukemia (CLL), respectively] can be easily recognized by using a series of cell-type marker proteins identified by comparison of fractionated normal cell populations. B and T cell types of CLL could be distinguished, the results correlating well with those obtained by use of monoclonal-antibody staining methods. In two cases representing almost pure B-cells (classical CLL; 0% T, 85% B) and T-cells (cutaneous T-cell leukemia; 77% T, 0% B), 27 of 29 marker proteins showed quantitative B/T differences comparable to those observed in comparisons of normal B-and T-lymphocytes prepared by cell sorting. These results indicate that cells from relatively well-differentiated leukemias show complex patterns of gene expression very similar to those of the corresponding normal cells and strongly support the use of large marker panels in cell-type determination. Less-well-differentiated acute leukemias [such as acute undifferentiated and acute granulocytic (FAB:M1)] appear to yield protein patterns corresponding less closely to recognizable mature cell types, and may show expression of novel proteins related to the state of differentiation.