In order to study the early events associated with infection of swine by Yersinia enterocolitica, 42 five-week-old crossbred piglets were inoculated per os with approximately 10(8) Y. enterocolitica O:3. Groups of 5 animals (and one negative control) were euthanized 30 min, 3, 6, 12, 24, 48 and 72 h following the infection. Palatine tonsils, retropharyngeal and mesenteric lymph nodes, esophagus, duodenum, jejunum, ileum (and Peyer's patches), stomach, liver, spleen and feces (from colon) were collected and analyzed for the presence of Y. enterocolitica by standard bacteriological procedures. Natural infections were also analyzed, as a complementary study, by taking one-gram samples of fecal material and tonsils from 291 pig carcasses less than 3 h after slaughter and culturing them for Y. enterocolitica using a cold enrichment technique. Within 30 min, Yersinia enterocolitica O:3 was already present at most sites. The presence of Y. enterocolitica in the liver of 3 out of 10 animals and also in the spleen of 3 out of 10 piglets, within the first 3 h postinfection, but not at later times (with one exception), probably indicated a transient bacteremia accompanying the initial stages of infection. The tonsils were colonized in most animals (13/20) as the bacteria remained present from 12 to 72 h postinfection, while only 4 out of 20 fecal samples were found to be positive over the same period. Up to 10(4) colony-forming units of Y. enterocolitica per gram of tonsil and fecal material were recovered. Finally, among the 291 animals sampled at the abattoir, a total of 79 were found positive, 70 of the tonsils sampled were positive, and bacteria were recovered in 17 fecal samples. It is therefore suggested that palatine tonsils are the most reliable tissue for the indication of an infection/colonization by Y. enterocolitica O:3 in swine and that the removal of this tissue during the slaughter process should be considered in order to minimize the possibility of contamination of meat products.