We have investigated whether cytolytic cells specific for hapten-coupled syngeneic cells have the same public idiotype as serum antibodies against the same hapten, NP-aminocaproyl (NP-cap). Anti-NP-cap antibodies of C57BL mice possess a public idiotype that can be detected by antisera against the whole V region of the antibody molecule or against the VH domain. Neither reagent had an effect on the cytolytic response either when they were used to sensitize the effector cells for complement killing or when used at high concentrations in the culture medium during the secondary stimulation in vitro.