Hybrid swine were immunized twice at an interval of 3 weeks to evaluate the efficacy of a liveS. typhimurium vaccine. The animals and a control group were challenged at the age of 8-10 weeks by oral test infection with a labelled S. typhimurium DT l 04 strain. An ELISA was used to establish the presence of antibodies to S. typhimurium in serum samples. The presence of the challenge strain in the ileal and caecal mucosa and in the ileocolic lymph nodes was investigated quantitatively using the Koch plating method. The vaccinated animals had significantly higher antibody titres after the second vaccination than the unvaccinated animals. The vaccinated animals had a significantly lower (p < 0.05) colonization of the ileal and caecal mucosa as well as the ileocolic lymph nodes than the unvaccinated animals.