Two strains of Salmonella enteritidis phage type 4 isolated from chickens, SeLA5 and Se267, were virulent in Balb/c mice by intraperitoneal (i.p.) infection. In 18-20-day-old chickens given 10(7) c.f.u. intravenously (i.v.), both strains caused some deaths. One-day-old chicks infected orally with 10(9) c.f.u. of either strain also suffered limited mortality. Strain SeLA5 was the more virulent by the oral route. AroA mutants SeLA5 aroA and Se267 aroA were reduced in i.p. virulence by up to six logs in the Balb/c mouse model, did not kill 18-20-day-old chickens when injected i.v. and did not multiply in the tissues. Oral vaccination of chicks with either 10(9) c.f.u. at one-day-old or 10(5) c.f.u. at seven-day intervals until day 21 with vaccine strains SeLA5 aroA or Se267 aroA conferred significant protection against intravenous challenge with virulent strain Se109 NalR and reduced intestinal shedding following oral challenge. Seroconversion following oral vaccination was not seen with the lower dose regimen and was low with the higher dose. Seroconversion following oral challenge was marked in unvaccinated birds but much lower in vaccinated birds, suggesting that oral vaccination had prevented invasion by the challenge strain.