Lysis of DNP-coated chicken erythrocytes by human blood lymphocytes (K cells) was induced by means of rabbit anti-DNP antibodies. Antisera were prepared by injecting the animals with DNP-conjugated proteins emulsified in Freund's complete adjuvant. An ammonium sulphate precipitation technique was used for assay of antibody concentration and affinity. Sephadex G-200 chromatography indicated that 90% of the DNP antibodies were 7S in the bleedings on days 10-16, whereas 99.8% were 7S in later bleedings. 7S antibodies induced K-cell lysis at high dilutions, whereas 19S antibodies were essentially negative. Antibody fractions obtained by DEAE- or CM-cellulose chromatography were used to establish possible heterogeneities in the capacity of 7S antibodies to induce either K-cell- or complement-mediated target cell lysis. No such heterogeneities were founnd. Fifteen IgG preparations containing antibodies of different affinities were compared with regard to their capacity to induce K-cell-mediated lysis. A statistically significant correlation was found between antibody affinity and efficiency in K-cell-mediated lysis. In a similar study of complement-mediated lysis the correlation was not significant at the 5% level but was significant at the 10% level.