We attempted to produce diarrhoea experimentally in the newborn calf by orally injecting 17 colostrum-deprived calves with two serotypes of Escherichia coli Ent+ K99+, a rotavirus and a coronavirus. With E. coli alone, a dose of 2 x 10(8) bacteria administered 24 hours after birth causes a mild attack of diarrhoea, whereas 1 x 10(10) bacteria leads to dehydration and death. An inoculation of rotavirus is followed by diarrhoea which always contains large quantities of rotavirus. These animals were anorectic for a time, but none was dehydrated or died. With coronavirus, there were large quantities of watery diarrhoea, which led to dehydration and death. The inoculation of rotavirus, not lethal in itself, followed by a similarly non lethal inoculation of E. coli in doses of 3 x 10(8) to 2 x 10(9) led to dehydration and death. The authors conclude that dehydration and death of the animal can caused by large doses of E. coli or coronavirus or by two non-lethal doses of rotavirus and E. coli administered one after the other.