The nation-wide salmonella control program is based on serological monitoring of swine herds (meat juice samples taken at the slaughter line) and classification of the herds on the basis of serology (Mousing eta!. 1997). When a herd is classified as a level 2 or level 3 herd (moderate or high sera-prevalence), a follow-up program is instituted. From August 1996, the follow-up program comprises: (1) a letter to the individual herd with information about the assigned salmonella level and the follow-up program (the injunction); (2) mandatory visits by veterinary advisors including a profile of the salmonella occurrence in the herd. The profile must comprise pen samples for bacteriological examination for Salmonella enterica.; (3) a plan for interventions; and (4) penalties if the herd does not comply with the program. On the submission forms to the Danish Veterinary Laboratory DVL the individual samples can be marked. Materials from swine are typically marked with sow number, stable, unit, or age-group.