Study A. Three experiments were performed to study the influence of removal of pigs at different ages prior to the age when S, typhimurium had been detected in subclinically infected herds. Study B. Based on these results, a model for Salmonella reduction on herd level was established. The model consisted of a microbiological survey in the herd to locate infected parts of the herd. Based on the microbiological results a plan for reduction of Salmonella was described for each individual farm. A typical plan consisted of hygienic measures combined with all in-all out measures on either pen-level or section level, in combination with an attempt to improve what we define as "enteric health" (absence of diarrhoea) in finishers without use of antibiotics. Study C. A cross-sectional study was performed using data from a data-base from a Danish slaughterhouse cooperative.