Serological studies for the presence of CFT antibodies to Actinobacillus pleuropneumoniae, both to serotype 2 and serotype 9, were done in twenty-five sows on each of thirty pig-breeding farms in the provinces of North Brabant and Overijssel. Antibodies to serotype 2 and/or serotype 9 were detected in one or several sows on forty-two out of sixty farms. Similar findings were recorded on eighteen and twenty-four farms respectively in the provinces of Overijssel and North Brabant; the number of herds showing positive reactions to serotype 2 was sixteen and twenty-three in these provinces respectively, that showing positive reactions to serotype 9 being six and five respectively. An average proportion of nineteen per cent of the samples taken on the farms having serologically positive animals was positive for serotype 2, and twelve per cent for serotype 9. Only one of the twenty-five animals on twelve farms was found to be positive; serotype 2 was present in ten cases and serotype 9 in two cases. A marked effect of the size of the herds on the serological status of the farms was not observed in the present studies. This was also true of the purchasing policy; there was no difference between farms which had no longer purchased any breeding pigs for an average period of seventeen years and for one year respectively. Clinical symptoms of an infection due to Actinobacillus pleuropneumoniae were only reported as occurring on two farms having serologically positive animals.