Heterogeneity of leukemic cells was defined in a case of lymphoma. Four phenotypically distinct subpopulations of leukemic cells were identified. One subpopulation was observed to simultaneously express B- and T-cell characteristics. B-cell characteristics included monoclonal IgM (lambda) surface immunoglobulin, HLA-DR antigens, and expression of the B-cell antigen identified by the BA-1 monoclonal antibody. T-cell characteristics included E-rosette formation, expression of the pan-T- associated antigens recognized by the Leu-1 and OKT-11 monoclonal antibodies, and expression of the suppressor cytotoxic T-cell- associated antigen recognized by the Leu-2 and OKT-8 monoclonal antibodies. In addition to this subpopulation, three other phenotypically distinct subpopulations were identified, two of which expressed monoclonal IgM (lambda) surface immunoglobulin. The results of this investigation indicates that three phenotypically distinct lymphoid subpopulations bearing B-cell characteristics, and probably a fourth T-cell subgroup, were derived from a common lineage. These findings suggest that the malignancy involved a lymphoid progenitor cell that may possess diverse maturational capacity.