Twenty-eight 10-week-old pigs were inoculated intratracheally with 1 × 105 colour-changing units/ml Mycoplasma hyopneumoniae strain 232, and another 32 pigs were not inoculated but were divided into 12 direct-contact pigs and 20 indirect-contact pigs. Thirty-five days later, the inoculated pigs were inoculated intranasally with 1 × 102·4 tcid50 of porcine reproductive and respiratory syndrome virus (prrsv) strain mn 30-100. Viraemia, seroconversion and the transmission of prrsv in the M hyopneumoniae-infected pigs were then assessed for four months. Three groups of 10 age-matched gilts were introduced as sentinels into the experimental barn on days 28, 56 and 84 after the prrsv infection. The persistence of prrsv was evaluated in both the experimentally and naturally infected pigs, which were slaughtered 120, 135 and 150 days after the infection. The period of viraemia and the extent of seroconversion were similar to those observed in studies of pigs infected only with prrsv, suggesting that under the conditions of the study M hyopneumoniae did not affect these features of the disease. A delayed pattern in the seroconversion and proportion of pcr-positive pigs was observed in the direct and indirect contact groups, and the persistence of prrsv in tissues was confirmed by pcr at 120 and 150 days after infection only in the directly inoculated pigs and not in the direct- or indirect-contact groups of pigs.