Monoclonal antibodies were raised against the leukemic T cells from a patient with chronic lymphocytic leukemia. Two antibodies, termed T411 and T811, were obtained which were reactive by indirect immunofluorescence only with cells of the T cell lineage. The T411 antibody recognized a polypeptide chain of 100,000 dalton apparent molecular weight which was present on the surface of 94 +/- 4% of peripheral blood T lymphocytes, but only on 20 +/- 8% of thymus cells. The antibody T811 reacted with a surface molecule composed of 2 poly-peptide chains of 32,000 and 34,000 dalton apparent molecular weight, which was expressed only on 25 +/- 10% of blood T lymphocytes and on 90 +/- 4% of thymus cells. Functional analysis of the T811+ and T811- T cell subsets isolated by rosetting with anti-mouse Ig coated ox erythrocytes revealed that both subpopulations were able to mount a proliferative response to allo-antigens, whereas allo-antigen induced cytotoxic cells and their precursors were only found in the T811+ subset. The pokeweed mitogen induced in vitro differentiation of B lymphocytes into immunoglobulin secreting cells was dependent on the presence of the T811- subset, whereas the T811+ T cells efficiently suppressed this differentiation.