Pigs in three specialized fattening herds were studied with respect to some immune functions, viral infections revealed by presence of interferon-alpha in serum as well as bacterial infections indicated by antibodies to Mycoplasma hyopneumoniae and Actinobacillus pleuropneumoniae. Pigs from different breeders had signs of varying but low incidence of such infections. After transport to the fattening herds, viral infections were activated and spread during the first month, and a maximum of 19% of the pigs had signs of ongoing infections at one week. At this time, blood leukocytes prepared from the pigs had diminished ability to produce interferon-alpha in vitro. Except during the first month, serological evidence of Mycoplasma hyopneumoniae infections became gradually more frequent, and 91% of the pigs were seropositive at slaughter. Early, but not late, infections appeared to inhibit growth of the pigs as indicated by time for slaughter, but only late infections may be registered as pneumonia at slaughter. The prevalence of pigs with serological evidence of Actinobacillus pleuropneumoniae infections increased late in the fattening period, but such infections had no demonstrable impact on the time for slaughter.