The vast majority of leukemic cells bear surface Ig. The class restriction of this surface Ig and other characteristics described for chronic lymphocytic leukemia (CLL) are considered as evidence for the monoclonality of the neoplastic proliferation. Different CLL cases with various expressions of surface Ig represent various degrees of block in the maturation of the lymphocytes. Through the use of other cell markers, it has been shown that most cases of CLL represent B cell neoplasia. Variations in the expression of the various B cell markers on the leukemic cells were observed. One case in which the leukemic cells were clearly T cell in character, and a single case of mixed B and T CLL, are described. The significance of the sheep red blood cell rosette-forming cells in CLL is discussed. Scanning electron microscopy (SEM) can be a useful adjunct to the identification of B- and T-derived lymphocytes. According to these criteria, leukemic cells are mostly of the B type, although variations in their surface architecture were noted. It is concluded that SEM alone cannot consistently distinguish between leukemic B and T cells. Electron microscope studies of mitogen-transformed CLL lymphocytes suggest that there is a residual normal B and T population of cells in addition to the predominant, abnormally reacting cells, which are mostly of B origin. Antigenic changes on the surface of CLL lymphocytes are suggestive of normal antigens present on young normal lymphocytes, rather than of the emergence of truly "tumor-specific" antigens. These antigens are independent of the T or B origin of the leukemia. CLL lymphocytes are shown to be defective in their ability to form caps with concanavalin A and anti-HL-A sera, and in their osmotic regulatory capacity.