Twenty gnotobiotic piglets were inoculated with 5 x 10(8) colony forming units of an Actinobacillus pleuropneumoniae biotype 1-serotype 9 strain onto their tonsils. Five other piglets (controls) were inoculated with phosphate-buffered saline solution. Pigs were euthanized at 30 min, 90 min, 180 min, 6 h, 9 h, 12 h or 24 h after inoculation. At necropsy, samples were taken from the tonsils for bacteriological, histological, immuno-histochemical and electron microscopical examination. A. pleuropneumoniae was isolated from tonsils of all the infected pigs, but not from tonsils of the control pigs. Early after inoculation bacteria were mainly associated with the stratified squamous epithelium and detached epithelial cells. Vacuolization and desquamation of the epithelium was observed and many transmigrating neutrophils were present. At later times after inoculation, bacteria were found closely associated with the crypt-walls and with detached cells present in the crypts. A strong neutrophil migration was observed mainly in the deeper parts of the crypts. It is concluded that attachment of A. pleuropneumoniae to tonsillar epithelial cells probably constitutes a first step in establishing bacteria at this body site.