Forty-five cold agglutinins (CA) were tested against various populations of lymphocytes by cytotoxicity and immunofluorescence assays. Marked differences were observed between anti-I and anti-i CA. Thirty-six per cent of anti-I killed preferentially peripheral blood and tonsillary B lymphocytes, whereas only 12 per cent killed preferentially T lymphocytes. Anti-I killed a much higher proportion of B-chronic lymphocytic leukemia cells than peripheral blood lymphocytes, peripheral blood B cells or T-chronic lymphocytic leukemia cells. Forty-three per cent of anti-i killed preferentially peripheral blood T lymphocytes and 54 per cent killed preferentially tonsillary T cells, whereas only 14 per cent killed more peripheral blood B cells and none killed preferentially tonsillary B cells. The kill of thymic lymphocytes and T-chronic lymphocytic leukemia cells by anti-i was very high, whereas the kill of B-chronic lymphocytic leukemia cells was very low. Almost all CA of other than I-i specificities showed preferential kill of peripheral blood B (83 per cent) and tonsillary B (67 per cent) cells. Tonsillary lymphocytes were usually more susceptible to the cytotoxic activity of CA than peripheral blood lymphocytes. Cold agglutinins with kappa light chains killed more B cells whereas CA with gamma light chains seemed to kill more T cells. Cytotoxicity did not correlate to the utilization of complement. It is suggested that the density and/or the accessibility of membranous antigens may be different on B and T cells, or alternatively that in addition to antigens common to all lymphocytes, anti-I and non-I/i cold agglutinins recognize specific antigenic determinants on B lymphocytes, whereas anti-i cold agglutinins recognize specific antigenic determinants on T lymphocytes.