The interaction of bacteria and virus has been well demonstrated in the pathogenesis of respiratory disease in swine. The interaction between porcine respiratory and reproductive syndrome virus (PRRSv) and Haemophilus parasuis has not been studied. We initiated studies to evaluate a possible effect of the PRRSv on the pathogenesis of polyserositis caused by H. parasuis. A group of 30 three week old piglets were distributed in 4 groups. Group I (10 pigs) was inoculated with PRRSv and H. parasuis. Group II (10 pigs) was inoculated with H. parasuis alone. Group III (5 pigs) was inoculated with virus alone and group IV (5 pigs) was inoculated with culture media. Lesions consisted of a severe fibrinous polyserositis affecting 7 of 10 animals in group II and a mild fibrinous pleuritis in 1 of 10 animals of group I. Three of ten animals dually infected with the two agents died during the course of the study. These animals had pulmonary congestion and focal lung hemorrhages. No other animals died from other groups. Group III and IV had no macroscopic lesions. Microscopically group III had interstitial pneumonia. Immunomodulating virus effect may explain the differences in terms of lesions severity between groups I and II. Septic shock was suspected as cause of sudden death.