Goats were inoculated with Pasteurella haemolytica A1 by three different routes to evaluate a caprine model of reproduction of pneumonic pasteurellosis. Lungs were examined and compared grossly, microscopically, and by bacterial culture. All of three goats inoculated intratracheally, one of three goats inoculated intravenously, and none of three goats inoculated intratonsillarly developed pneumonic lesions. Pneumonic lesions in the intravenously-inoculated goat were similar in gross and microscopic aspects to the lesions present in the lungs of intratracheally-inoculated goats. In a separate study, there was evidence of replication of P. hemolytica organisms in the lungs of cattle after subcutaneous vaccination with a live P. haemolytica vaccine, and evidence that the organisms may have gained access to the lung via the vascular system;Humoral immune responses of cattle to P. haemolytica have been thoroughly studied, and humoral immune responses sometimes do not correlate with protection against experimental disease. Other factors, including cell-mediated immune responses, may be involved in protection against pneumonic pasteurellosis. To evaluate the cell-mediated immune responses to P. haemolytica, calves were vaccinated with a commercial P. haemolytica bacterin, or a commercial live P. haemolytica vaccine, or they were infected intratracheally with P. haemolytica. Lymphocyte cultures from peripheral blood mononuclear cells or lymph node cells were stimulated with antigens prepared from P. haemolytica to evaluate in vitro lymphocyte proliferative responses and gamma interferon production as measures of cell-mediated immunity. In response to stimulation with an outer membrane protein preparation from P. haemolytica, strong proliferative responses and gamma interferon production were detected in lymph node cells from calves vaccinated with the live vaccine and from infected calves;All calves were challenge-exposed to P. haemolytica, and cell-mediated immune and humoral immune responses were correlated with the degree of pneumonia after experimental challenge. Neither of the cell-mediated immune responses nor the humoral immune response was strongly correlated with the degree of pneumonia present after the experimental challenge.