Lesions of calf pneumonia caused by Pasteurella haemolytica or P. multocida were examined histopathologically using an immunoperoxidase technique for the detection of the bacterial antigen. The results indicated that:1) multifocal coagulation necrosis sharply demarcated by degenerated leukocytes was a characteristic pulmonary lesion caused by P. haemolytica, 2) other bacteria than P. haemolytica sometimes caused necrotic lesions in lungs of calves with naturally acquired pneumonia, 3) the lack of multifocal coagulation necrosis in P. multocida pneumonia was the major difference from P. haemolytica pneumonia, and 4) the immunoperoxidase technique can be used for the accurate diagnosis of pneumonia in cattle.