B- and T-cell markers were studied in a patient with chronic lymphocytic leukemia and erythroderma. The absence of immunoglobulin, complement receptor, and Fc receptor, and the presence of sheep erythrocyte receptor and T-cell antigen on the membrane of the leukemic cells classified them as thymus derived. Using quantitative microphotometric immunoautoradiography, surface antigen densities were measured at the cellular level with the following results: (1) The density of T-antigenic sites was less on leukemic cells compared to normal T lymphocytes. (2) The T-antigen densities of leukemic lymphocytes varied less from cell to cell forming a homogeneous peak in histograms. (3) An Ig density of normal B lymphocytes was demonstrated on the residual T-antigen-negative cells. The results were qualitatively confirmed by direct immunofluorescence and electron microscopy with peroxidase-labeled antibodies. Furthermore, the surface antigens were quantitative microcomplement fixation test which revealed reduced binding of anti-T-cell antibodies and complement, and no antiglobulin fixation on the leukemic lymphocytes. Since lymphocytes with normal T-antigen concentration could not be found among the leukemic T lymphocytes, a lack of normal T cells was assumed. The findings that there was a decrease and altered distribution of surface markers on chronic lymphatic leukemia cells of the B- and T-cell type are discussed as further arguments referring to their clonal origin.