All the animals positive at 6 weeks of age were negative at the next sampling occasion. Therefore, we considered these pigs seropositive due to the presence of maternal antibodies and not due to infection with M. hyopneumoniae. In herds 1 and 2, the first seropositive pigs due to contact with M. hyopneumoniae were found at 10 weeks of age, but less than 25% of the pigs were seropositive at 20 weeks. In herds 3 and 4, sero positivitydue to infection with M. hyopneumoniae was found later namely at 14 and 20 weeks of age, respectively, but the majority of the pigs were seropositive at 20 weeks of age.