Human peripheral blood leukocytes are cytotoxic to 51Cr-labeled target cells provided phytohemagglutinin is present in the reaction mixture. The lectin-dependent cellular cytotoxicity (LDCC) phenomenon is characterized by (1) the short duration of the assay (2 to 4 hr), (2) revealing potential cytotoxic cells present in the cell preparation and excluding participation of mitogen-induced polyclonal activation of effector cells generated during prolonged assays used in mitogen-induced cytotoxicity, and (3) using nonerythrocyte target cells. The effector cells that mediate LDCC were characterized by several cell fractionation procedures. Fractionation of Ficoll-Hy-paque separated peripheral blood leukocytes on polystyrene bead columns or on nylon fiber columns which remove monocytes, granulocytes, and immunoglobulin-bearing leukocytes did not remove the LDCC activity. Polymorphonuclear leukocytes preparations were devoid of LDCC activity. Enrichment for T cells by E rosette sedimentation demonstrated that LDCC activity is present in both the T cell-enriched fraction and the non-T cell fraction. Nylon column eluted non-Ig-bearing cells which were subsequently depleted of T cells by E rosette separation were LDCC positive. Depletion of Fc-bearing leukocytes on EA monolayers removed the LDCC activity. These results demonstrated that two populations of Fc-bearing cells of thymus and nonthymus origin mediate LDCC. Normal and malignant cells of immune and human origin were good targets in LDCC. The wide range of sensitive target cells in LDCC makes this test feasible in a completely autologous or syngeneic system. The significance of the LDCC phenomenon in assessing cell-mediated immunity is discussed.