The ability of sheep leukocytes to mediate antibody — dependent cell-mediated cytotoxicity (ADCC) and that of sheep serum IgG1 and IgG2 to induce ADCC were investigated. Partial characterization of effector cells was attempted. These investigations revealed that ADCC occurs in sneep. With chicken erythrocytes (CRBC) as the target cells, polymorphonu-cleated cells (PMN), and monocytes, were the most effective leukocytes. Ovine peripheral blood lymphocytes (PBL) also mediated ADCC, and within the PBL population, T-cells were capable of mediating ADCC. The T-cells were obtained by nylon wool fractionation and selective agglutination by peanut agglutinin (PNA) and Helix pomatia agglutinin (HPA). Both nylon wool adherent and non-adherent fractions were active in ADCC, although the former were more active, implying heterogeneity in nylon wool adherence among ovine K-cells. Depletion of B (SIg+) cells did not affect ADCC activity of the remaining cells. Depletion of Fc+ cells markedly reduced cytotoxic activity of PBL. Both sheep IgG1 and IgG2 anti-CRBC immunoglobulins were able to induce ADCC.