Surface marker techniques have made a major contribution to the understanding and classification of lymphoproliferative disorders by permitting the determination of B- and T-cell lineage. The frequent malignant proliferations of B lymphocytes are identified by the presence of surface immunoglobulin of a single light-chain type; Ia-like (HLA-DR) antigen is present as well. While most T-cell proliferations exhibit the classic receptor for sheep erythrocytes, commercially available monoclonal antisera permit the secure identification of T cells and their subclassification into inducer-helper and cytotoxic-suppressor subsets. Surface markers have also allowed the separation of a fraction of patients with acute lymphocytic leukemia whose cells have T-lymphocyte markers from the majority whose cells show subtle evidence of early B-cell differentiation.