Circulating peripheral blood tumor cells in four cases of chronic lymphoproliferative disease were immunologically characterized. By the use of T-cell-specific heteroantisera and indirect immunofluorescence, all were shown to involve proliferation of malignant T cells. Three cases demonstrated morphologic and clinical features consistent with chronic lymphocytic leukemia (CLL), and one case presented as a lymphosarcoma cell leukemia. Antisera specific for normal human T-cell subsets defined the malignant T cells in each case as arising from the TH2--subset. This subset normally constitutes approximately 80% of human peripheral blood T cells. Terminal deoxynucleotidyl transferase (TdT) was not detected in any of the T-cell CLL cases, thus supporting the notion that T-cell CLL represents a malignancy of a mature phenotype. The one patient with lymphosarcoma whose tumor cells were TdT-positive subsequently developed T-cell acute lymphoblastic leukemia (ALL). Moreover, la-like antigen (p23,30) was detected on two of these tumor cell populations. In addition, it was shown that not all tumor cells were E-rosette-positive, since only cells from 3 of 4 patients were capable of forming spontaneous rosettes. These findings demonstrate that heteroantisera can provide an additional important tool for dissecting the heterogeneity of T-cell leukemias and for relating them to more differentiated normal T cells.