An in vivo loop test model for the investigation of the invasiveness of Salmonella enterica in chickens was developed. Ten jejunal loops were made in 10- to 12-week-old Lohman Brown chickens under isofluoran anaesthesia. Salmonella at 5.0 × 10⁷ CFU was inoculated into each loop and left for 2 h, followed by a 1-h incubation with gentamicin in order to kill noninvading bacteria. After euthanasia, Salmonella invasiveness was measured as tissue-associated counts relative to a reference strain. The ability of Salmonella invasion was 1 log₁₀ CFU higher per 42-mm² mucosal tissue in the anterior than in the posterior part of jejunum. A statistically significant (P < 0.001) sixfold difference in invasiveness was observed between a wild-type S. enterica serotype Typhimurium strain and the corresponding invH mutant. The model was shown to be able to show small differences in invasive capability and allows for comparison of strains tested in different animals, provided that the same reference strain is present in all animals.