To investigate the effect of systemic neutrophil depletion on ovulation rate, rats were synchronised with eCG and hCG, and concurrently were administered neutrophil-specific, cytotoxic RP-3 monoclonal antibody (mAb), or an irrelevant, class-matched mAb. Neutrophils in the peripheral blood and in the thecal-luteal area of corpora lutea were detected by immunohistochemical analysis with the neutrophil-specific mAb MCA149 and were found to be reduced in number by 70% and 38% respectively following RP-3 treatment compared to the control group. Ovulation rate, as assessed by counting the number of oocytes in the ampullary region of the oviduct 20 h after hCG administration, was found to be reduced by 27% in the neutrophil-depleted rats. This result provides further evidence that neutrophilic granulocytes play an active role in ovulation in the rat.