NIP-conjugated Fab' fragments from a rabbit hyperimmunized with sheep red blood cells (SRBC) were used to link the hapten NIP to target cells for antibody-dependent cell-mediated cytotoxicity (ADCC) and antibody-dependent complement-mediated lysis (ADCML). Target cells (SRBC) labelled with this NIP-Fab' complex were compared with SRBC directly haptenized with NIP in ADCC and ADCML assays using a NIP specific IgG1 chimeric antibody. Both methods yielded almost identical results. Using the NIP-FAb' conjugate identical target cell haptenization was readily achieved from experiment to experiment. Using conjugates of different NIP/Fab' ratios it should be possible to study how such changes influence antibody effector functions.