An attenuated derivative of the fully calf virulent Salmonella typhimurium strain ST4/74 was constructed by introducing stable mutations into genes aroA and aroD rendering the strain dependent for growth on aromatic compounds. The strain (BRD562) was used to vaccinate orally ten Friesian calves 7 days after birth with approximately 10(10) live organisms. The only effects of vaccination in the calves was a transient faecal excretion of the organism, mild transient diarrhoea and mild pyrexia in one animal. Two of these animals were tested for the presence of BRD562 at necropsy 56 days after birth but none were shown to be infected. Eight of the vaccinated calves were subjected to an oral challenge infection with ST4/74 28 days after vaccination and seven out of the eight animals survived. The only effect of challenge in the surviving calves was mild pyrexia and transient excretion of ST4/74 in their faeces; only one of the seven animals was found to be infected at necropsy 28 days after challenge. The eighth challenged calf scoured severely and was killed in extremis 7 days after challenge. Four unvaccinated control calves infected orally with ST4/74 28 days after birth all scoured severely and were euthanased for humane reasons 5-7 days later. This study suggests that vaccination with an attenuated strain of S. typhimurium harbouring mutations in two aro genes is a safe and effective way of protecting young calves against experimental S. typhimurium infection.