Oral immunisation using a feed-incorporated vaccine comprising heat inactivated Escherichia coli and Salmonellae antigens was examined in feeding trials with calves. The studies were conducted with batches of calves purchased at intervals throughout one year and reared in an environment created by continuous occupancy. Improved health was registered in terms of a significant reduction in the incidence and duration of diarrhoea (P less than 0.01) and in the need for treatment with medicament or antibiotics (P less than 0.01). Orally immunised calves showed a significantly better weight gain (1.44 kg, P less than 0.05) than the controls during the period 0 to 5 weeks. This gain was mainly obtained during the first three weeks when the antigen fed calves showed significantly improved faeces scores and a significantly better appetite for concentrates (P less than 0.05).