Serological screening for the presence of antibodies to both H. pleuropneumoniae serotype 2 and serotype 9 using the complement fixation test was carried out on twenty-nine piglets production farms of an integrated group in the province of North Brabant. Antibodies to H. pleuropneumoniae serotype 2 and serotype 9 were not detected on six of the twenty-nine farms when a single screening was performed. The proportions of farms with animals serologically positive in regard to serotypes 2 and 9 were 59 and 34 per cent respectively. Of the specimens on farms with seropositive animals, those positive for serotype 2 averaged 13 per cent, those positive for antibodies to serotype 9 averaging 9 per cent. When a re-examination was made within two months after the first screening, animals showing antibodies to serotype 2 or serotype 9 were present on three of the six pig-breeding farms which were negative for these antibodies at first examination. When a third investigation was carried out within two months after the re-examination, antibodies to serotypes 2 or 9 were also detected on the remaining farms which were negative for antibodies in the two previous investigations.