Peripheral blood neutrophils from Rocky Mountain bighorn sheep (Ovis canadensis canadensis) and domestic sheep were exposed to culture supernatants from Pasteurella haemolytica isolates recovered from these two sheep species. Six culture supernatants from bighorn sheep isolates and two from domestic sheep isolates were tested for cytotoxicity as determined by the release of lactate dehydrogenase. Two of the bacterial culture supernatants from bighorn sheep were not cytotoxic, while the other four bighorn sheep culture supernatants were effective cytotoxins on both bighorn (> 95% cell death at 150 micrograms of cytotoxin) and domestic sheep neutrophils (55 to 95% cell death at 150 micrograms of cytotoxin). Two culture supernatants of P. haemolytica from domestic sheep were effective cytotoxins on both bighorn (> 95% cell death at 150 micrograms of cytotoxin) and domestic sheep (70 to 75% cell death at 150 micrograms of cytotoxin) neutrophils. Potency of cytotoxins derived from P. haemolytica isolates from bighorn sheep was three to seven-fold higher when tested with bighorn sheep neutrophils as compared to domestic sheep neutrophils. Cytotoxins derived from P. haemolytica isolates from domestic sheep were five to six-fold more potent when tested with bighorn sheep neutrophils than when domestic sheep cells were used.