In the present study, a total of 235 buffalo calves (1-8 months of age) from different localities were investigated. They were grouped, 80 apparently healthy calves (group 1). 105 with respiratory manifestations (group 2) and 50 dead calves suffered from respiratory manifestations before death (group 3). Nasopharyngeal swabs were taken from group (1 and 2) and lung tissues, lymph nodes and liver from the groups (2 & 3) for detection of P. multocida and specimens from the lung, associated lymph node and liver were taken from each group for the histopathological studies. P. multocida was isolated from 6.25%, 10.47% and 66% of each calf group respectively. The isolated P. multocida were pathogenic to mice and belonged to serotype B:2.The histopathological studies revealed fibrinous bronchopneumonia, severe pleuritis, hyperplasia in the lymphoid follicles of the lymph node and fibrinous hepatitis of group (2); hemorrhagic pneumonia with adhesive pleuritis, depletion of the lymphoid follicles of the lymph node and hepatitis were seen in calves of group (3). Heat labile toxin from cell sonicate of the isolated P. multocida serotype was purified by ammonium sulphate precipitation followed by ion exchange of gel filtration chromatography and poly- acrylamide gel electrophoresis. Toxic activity was assayed during toxin purification by intranasal insillation in rabbits. The most toxogenic band was the 150 KDa fraction as determined by SDS PAGE. Western blot analysis, rabbit antisera react with 150 KDa. Rabbits inoculated I/N with the purified toxin dead within 2 days. Showing symptoms of septicaemia with hemorrhagic pneumonia and pleuritis. Non of the rabbits inoculated with the heated toxin at 70°C for 30 minutes dead or had clinical sings of toxicosis. Pasteurella free rabbits were immunized I/N and I/conjunctival twice with 2 weeks interval and challenged I/N with a homologous P. multocid. For 2 weeks after the second immunization. The immunized challenged rabbits were shown to be protected against clinical sings of the disease and death with production of serum immunoglobulin against P. multocida. The lesions seen were slight emphsyma and few inflammatory cell infilteration of the lung. Mild depletion of the lymphoid follicle of the lymph node with mild hepatitis. In contrast, the non-immunized challenged rabbits dead within 4 days with heamorrhagic pneumonia, pleurtitis and hepatitis. Immunization with the purified toxin prevented all clinical sings of the disease and deaths.