AbstractSurface markers, terminal transferase, cytochemical staining, and function of peripheral blood lymphocytes were examined in nine patients with T‐cell lymphoproliferative disease. Anti‐T‐cell antiserum proved useful in characterizing the phenotype of the abnormal cells when only a minority of the abnormal cells formed E‐rosettes. Anti‐B‐cell antiserum was consistently unreactive with the abnormal cells. Terminal transferase was elevated in T‐cell ALL but low in T‐cell CLL, lymphoma‐leukemia, or Sézary syndrome. Lymphocyte proliferative response to nonspecific mitogens was generally depressed as was the response to allogeneic cells. The cells also failed to stimulate in mixed lymphocyte culture, implying a lack of Ia antigens on their surface. Acid phosphatase stain was positive in the three patients tested. Peroxidase and esterase stains were negative, whereas PAS was positive in five of eight patients tested. Prognosis in general, regardless of the clinical picture, (ie, acute, chronic, or lymphoma‐leukemia), was poor with seven of the nine patients dying within 14 months of diagnosis. Therefore, the use of anti‐T‐cell antisera and the E‐rosette surface marker should be used to characterize the phenotype of the malignant cells in lymphoproliferative disease to enable more vigorous treatment of those that are of T‐cell origin and that ther