Formalin-inactivated and sonicated Pasteurella haemolytica bacterins were prepared from 1 h cultures of bacterial pellets in RPMI-1640 medium containing 7% fetal calf serum. The bacterial pellets were obtained from logarithmic phase growth of the organism by centrifugation. The protective effect of the vaccine was evaluated in 43 specific-pathogen-free Hereford crossbred calves and yearlings in three experiments. Cattle were either single vaccinated or boosted via three routes; intratracheally (i.t.), intranasally (i.n.) or intramuscularly (i.m.), using low or high doses. The two low-dose groups were also given supernatant by the same routes and volume as the bacterin. Cattle were challenged by P. haemolytica in aerosol at 24 or 39 days after last vaccination. To enhance the susceptibility of the cattle to this challenge, the cattle were exposed to bovine herpesvirus-1 aerosol 4 days before the bacterial challenge. The extent of pneumonia was significantly less in three groups of cattle (i.n.-i.n.,i.m.-i.n.,i.m.-i.m.) boosted with high dose of the bacterin than in the controls. Protection was observed when challenge isolates were heterologous or homologous to the isolates used to prepare the bacterins. It was also observed that the level of complement fixing antibody or anticytotoxin activity to P. haemolytica did not correlate with the degree of protection.