The exacerbating effect of Mycoplasma hyopneumoniae (Mh) infection on the development of pneumonia caused by subsequent infection with Haemophilus pleuropneumoniae (Hp) was investigated in pigs. Pigs were inoculated intranasally with 10⁷ cells of Mh, followed 16 days later by an intranasal inoculation of 10⁵ cells of Hp. Other pigs were given either organism alone by the identical manner. One pig infected with both agents died 3 days after Hp inoculation, and had necrotizing, hemorrhagic fibrinous pleuropneumonia. All other pigs were killed 35 days after Mh inoculation. Pigs infected with both organisms showed more severe clinical signs, more extensive and widespread pneumonic lesions characterized by chronic form with necrotic nodules and pleurisy when compared with pigs infected with either organism alone. Extrapulmonary dissemination of both organisms occurred more frequently in doubly infected pigs than in pigs infected with either agent alone. These results indicate that previous infection of Mh acts as one of predisposed factors exacerbating porcine haemophilus pleuropneumonia. There were no correlations between antibody titers to Hp and intensity of lung lesions.