Our objective was to identify pigs of an endemically infected herd that were susceptible to pleuropneumonia due to Actinobacillus pleuropneumoniae. The presence of toxin-neutralizing antibodies was studied in serum of 36 pigs from birth until 24 weeks of age. Titers gradually declined during the first twelve weeks of life and increased thereafter. Sera from one-hundred 3-weeks-old piglets and one-hundred 20-weeks-old pigs were sampled and neutralization titers were determined. From each group we selected 5 pigs with the lowest titers and 5 pigs with the highest titers. These selected pigs (n = 20) were inoculated endobronchially with A. pleuropneumoniae. Pigs that survived from infection were necropsied after 48 h. Pigs with low neutralization titers had severe lung lesions, whereas pigs with high titers had no or minor lung lesion. These differences were significant (P < 0.05). From this field study we conclude that susceptibility to Actinobacillus pleuropneumoniae can be predicted by absence of toxin-neutralizing antibodies.