A model was developed to simulate the lateral spread of Salmonella enteritidis infection among chickens. One group of newly hatched chicks was vaccinated orally with S enteritidis aroA. At three weeks old naive chickens were infected with a wild-type strain of S enteritidis and brought into contact with separate groups of aroA vaccinated chickens and unvaccinated control chickens. The vaccinated chickens were well protected against colonisation of the gut by the wild-type strain whereas the control group became heavily colonised. The IgG responses to a lipopolysaccharide extract of S enteritidis in the vaccinated chickens indicated a limitation of invasion from the gut. Chickens vaccinated orally at one day old with S enteritidis aroA were not protected against oral or intravenous challenge at eight weeks old with a wild-type strain of S typhimurium. A group of newly hatched female chicks was vaccinated orally with S enteritidis aroA and again at two weeks old. A second group also received oral booster doses at 16 and 18 weeks. When challenged intravenously with a wild-type strain of S enteritidis at 23 weeks old there was a significant reduction in the numbers of this strain in the spleens, livers, ovaries and caeca of both vaccinated groups. Booster vaccination at 16 and 18 weeks of age induced the greatest protection of the caeca.