The results indicate that 15 of the 16 SPF (specific pathogen free) herds examined may be free from Y enterocolitica O:3. In a broad perspective this investigation Indicates that it is possible to establish cluster of pig herds free from Y. enterocolitica O:3, and to keep the herds free from the bacteria for many years. According to serological testing, the basic herd at the top of this SPF pyramid seem to have been free from this pathogenic variant since 1996. A total of 14 herds are confirmed negative also by culture.