Ten healthy, mature ponies were orally infected with Ehrlichia risticii using Ehrlichia-infected P388D1 mouse monocyte tissue culture cells. Seven developed signs of equine ehrlichial colitis including fever, depression, anorexia, reduced borborygmi, increased abdominal hyperresonance, and diarrhoea. Three remained clinically normal apart from early fever in one. Indirect fluorescent antibody titres were detected in the clinically affected ponies by Days 12 to 17 post infection and increased rapidly to high levels (1:640 to 1:5120) which were maintained until the end of the observation period (Day 28 post infection). Mean antibody titres significantly increased from Days 12 to 14 and from Days 14 to 17 post infection. The rapid increase in antibody titre which occurs close to first appearance of clinical signs may explain the insignificant rise in antibody titres in many naturally occurring cases.