Between February and April, 1994, we tested the hypothesis that bighorn sheep (Ovis canadensis canadensis) inoculated with a cytotoxic isolate of Pasteurella haemolytica biotype A, serotype 11 (All) could withstand challenge inoculation with a cytotoxic strain of P. haemolytica A2 of domestic sheep origin known to cause lethal pneumonia in bighorn sheep. On experimental day 0, two bighorn sheep were inoculated intratracheally with 6 × 10⁹ colony forming units (cfu) of a cytotoxic strain of P. haemolytica All (group 1); two bighorn sheep were inoculated intratracheally with 6 × 10⁹ cfu of a noncytotoxic P. haemolytica All (group 2), and two control bighorn sheep were inoculated intratracheally with a similar volume of brain heart infusion (BHI) broth (group 3). After inoculation, all bighorn sheep remained healthy. On experimental day 16, group 1 bighorn sheep each were given the same intratracheal inoculation as on day 0, and groups 2 and 3 bighorn sheep each were inoculated with BHI broth at the same volume as group 1. All bighorn sheep remained healthy following inoculations. On experimental day 42, bighorn sheep in groups 1 and 3 each were challenged with an intratracheal inoculation of 6 × 10⁹ cfu of P. haemolytica A2 of domestic sheep origin known to be lethal in bighorn sheep. Group 2 sheep each were inoculated intratracheally with BHI broth at the same volume as groups 1 and 3. The four bighorn sheep in groups 1 and 3 that received the challenge inoculation died from acute bronchopneumonia within 72 hours after challenge inoculation, and cytotoxic P. haemolytica A2 was isolated from the four dead bighorn sheep. Both cytotoxic or noncytotoxic strains of P. haemolytica All were not lethal and did not cause pneumonia in the experimentally inoculated bighorn sheep. However, previous inoculation with cytotoxic P. haemolytica All did not protect the bighorn sheep against later experimental challenge inoculation with a known lethal strain of cytotoxic P. haemolytica A2 under the conditions defined in these experiments.