Our previous work has shown that antibody-coated mouse spleen cells express enhanced cytotoxic activity against some Fc-receptor-bearing target tumour cells by a mechanism which appears to be similar to an antibody-dependent cell-mediated cytotoxicity (ADCC) reaction with reversed polarity of the antibody bridge (R-ADCC). In this report we have shown that (i) the levels of basal natural killer (NK), ADCC and R-ADCC cytotoxic activities in mouse spleen cells are strongly correlated with each other, (b) simultaneous induction of ADCC and R-ADCC reactions does not result in an additive cytotoxic response, and (iii) YAC cells which do not bear Fc receptors and are highly sensitive to lysis by NK cells, can specifically and competitively inhibit the ADCC and R-ADCC reactions. These results suggest that the R-ADCC reaction may be mediated by the same effector cell population as mediates NK and ADCC reactions against tumour target cells.