This study examined whether an infection with porcine reproductive and respiratory syndrome virus (PRRSV) potentiates respiratory signs upon exposure to bacterial lipopolysaccharides (LPS). Five-week-old conventional pigs were inoculated intratracheally with the Lelystad strain of PRRSV and received 5 days later one or two intratracheal LPS administrations. The necessary controls were included. After LPS administration, pigs were intensively monitored for clinical signs. Additionally, some pigs were euthanatized after a second LPS administration for broncho-alveolar cell analysis and virological examinations of the lungs. Broncho-alveolar lavage (BAL) cells were counted and differentiated. Lung suspensions and BAL fluids were titrated for PRRSV. Exposure of pigs to PRRSV only resulted in a fever for time periods ranging from 1 to 5 days and slight respiratory signs. Exposure of pigs to LPS only resulted in general signs, characterized by fever and depression, but respiratory signs were slight or absent. PRRSV-LPS exposed pigs, on the other hand, developed severe respiratory signs upon LPS exposure, characterized by tachypnoea, abdominal breathing and dyspnoea. Besides respiratory signs, these pigs also showed enhanced general signs, such as fever and depression. Lung neutrophil infiltration was similar in non-infected and PRRSV-infected pigs upon LPS exposure. PRRSV quantities were similar in lungs and BAL fluids of pigs infected with PRRSV only and PRRSV-LPS exposed pigs. These data show a clear synergism between PRRSV and LPS in the induction of respiratory signs in conventional pigs. The synergism was observed in 87% of the pigs. So, it can be considered as reproducible and may be used to test the efficacy of preventive and therapeutic measures.