The paper describes the clinical, bacteriological and pathological findings in experimental Salmonella typhimurium infection in calves. Oral doses of 10(8) and 10(9) organisms produced clinical disease and high mortality; doses ranging from 10(4)--10(7) organisms were less consistent in their action. Jersey calves appeared more susceptible to infection than Friesian calves. The clinical signs in most calves were pyrexia and a characteristic diarrhoea that lasted for up to 11 days; more severe symptoms were seen in the calves that received the higher doses. Following infection, all calves excreted S typhimurium in their faeces, the highest counts being observed in the calves that died. In the calves that survived, counts ranging from 10(2)--10(5)/g faeces occurred continuously for up to a maximum of 20 days and subsequent intermittent excretion occurred in a number of calves. In the calves that died, necrotic enteritis in the ileum and large intestine was the most striking lesion; lesions were uncommon in other organs. The findings are discussed in relation to the pathogenesis, diagnosis and control of the disease.