Abstract: Antibodies are retargeted to a target for which they have no functional specificity under normal circumstances. Use is made of a multi-specific binding substance which has binding specificity for the target and anti-antibody binding specificity. The binding substance may comprise an immunoglobulin antigen binding site and may be a “diabody”. Depending on the antibody bound, effector functions such as Complement, ADCC and immune blocking are recruited to act on the target. Example targets are human cells. In vivo and in vitro utilities are exemplified, including lysis of tumor cells and agglutination of red blood cells.