Sero-epidemiological studies were carried out in pigs aged 1 to 24 weeks in three herds in which Actinobacillus pleuropneumoniae was endemic. The sera were tested in the complement-fixation test and for their ability to neutralize the haemolytic and cytotoxic activities of the A. pleuropneumoniae serotypes isolated from the herds. Almost all (98%) sera from 1-week-old piglets neutralized the haemolytic and cytotoxic activities but only 21% fixed complement. At the end of the finishing period, most pigs (82%) had sera that neutralized haemolytic and cytotoxic activities and only 22% fixed complement. In longitudinal studies the neutralization titres decreased during the first 12-13 weeks of age. Thereafter, 75% of the pigs had increased titres in the haemolysin- and cytotoxin-neutralization tests and only 5% of the pigs had increased titres in the complement-fixation test. In none of these pigs were clinical signs of pleuropneumonia seen. Thus in these endemically infected herds the prevalence of complement-fixing antibodies was low, whereas the prevalence of neutralizing antibodies was high. The fact that serum-neutralizing titres are low in 12-week-old pigs might be the reason that pigs of this age are the most vulnerable to the disease.