The recombinant leukotoxin (rLKT) of the bacterium Pasteurella haemolytica A1 was examined for its ability to protect cattle from experimental challenge with logarithmic-phase P. haemolytica. Six different vaccines were utilized in the experiment: P. haemolytica culture supernatant, P. haemolytica culture supernatant enriched with rLKT, rLKT alone, P. haemolytica culture supernatant enriched with Escherichia coli supernatant not containing LKT, E. coli supernatant alone, and phosphate-buffered saline. rLKT alone showed no protective capacity against development of clinical signs of respiratory disease or against development of postmortem lung lesions after experimental challenge. It was, however, shown to enhance the efficacy of the culture supernatant vaccine and decrease clinical signs and pneumonic lesions. The complexity of protective immunity in this disease is emphasized in this study, and, although LKT is an important virulence factor of the organism, an immune response to LKT alone does not protect animals against disease.