Lymphocytes from patients with acute and chronic T-cell malignancy or chronic T gamma lymphocytosis were characterized by studying the activity of three enzymes involved in purine metabolism and by determining the isoenzyme pattern of lactate dehydrogenase (LDH) in addition to analysis of surface marker expression with monoclonal antibodies. Four clinically different types of disease were distinguished on the basis of the enzyme parameters. Lymphocytes from patients with acute lymphocytic leukemia (T-ALL) showed an enzyme profile similar to that of normal thymocytes, i.e., an elevated level of adenosine deaminase (ADA) activity as compared with normal T lymphocytes, reduced activities of purine 5'nucleotidase (5'NT) and purine nucleoside phosphorylase (PNP), and a binomial distribution of the LDH isoenzyme pattern. Cells from "null"-ALL patients had an ADA/PNP ratio that was intermediate between that of normal T cells and that of T-ALL cells or thymocytes, but their 5'NT activity and LDH isoenzyme pattern were thymocyte-like. Patients with chronic T-cell proliferation were subdivided into those with chronic T gamma lymphocytosis and those with proven chronic T malignancy. The lymphocytes from these patients had ADA and PNP activities within the ranges of those of normal T lymphocytes. However, the ADA activity and/or the ADA/PNP ratio were consistently higher in the cells from the patients with chronic T gamma lymphocytosis than in those with chronic T malignancy. The enzyme profile of the cells from the T gamma patients was similar to that of T gamma cells of normal individuals. The cells from patients with chronic T malignancies showed a heterogeneous enzyme pattern as compared with that of normal T lymphocytes. Analysis with monoclonal antibodies enabled us to distinguish null-ALL patients from the other leukemias studied, but a distinction between chronic and acute T-cell proliferation disease, for instance, was not possible with monoclonal antibodies alone. Our data demonstrate that the enzyme profiles studied provide supplementary information for classification and diagnosis of lymphoproliferative diseases to that obtained with cell surface markers alone.