Multi-resistant Salmonella typhimurium DTI04 (DT104) was first isolated from Danish livestock in 1996. DTI 04 is now known as an important and emerging pathogen in many countries. In Denmark, multi-resistant Salmonella typhimurium DT 104 is officially declared an unwanted bacteria in any food. Consequently, when DT 104 is detected in pork at a Danish slaugtherhouse, all carcasses from the day of slaugther in question must be heat treated. In order to find DT I 04 infected swine herds, a mandatory tracing back procedure is performed by strategical use of serology and culture.