The occurrence of diarrhoea in calves was monitored during the first three weeks of life. Calves fed amounts of colostrum sufficient to produce serum Ig levels in excess of 30 mg per ml did not develop diarrhoea, whereas calves fed less colostrum did. Rotaviruses and mycoplasma-like particles were observed in the faeces of calves with and without diarrhoea. The epidemiology of rotavirus infection in calves is discussed.