Three groups of four calves were used to determine whether Ehrlichia phagocytophila could be transmitted orally to calves via infected milk. Groups 1 and 2 consisted of four-week-old calves and group 3 of newborn calves. The calves in group 1 were fed for several days with milk from cows infected experimentally with E phagocytophila. The calves in groups 2 and 3 were fed 200 ml of whole blood containing E phagocytophila organisms; for group 2 the blood was added to milk before being fed, and for group 3 the blood was added to colostrum before being fed within three hours after birth. Blood samples for haematological, serological and cytological examination, and for polymerase chain reaction (PCR) were collected from all the calves, starting on the first or only day of administration and then every four days for four weeks. The calves of groups 1 and 2 showed no clinical, haematological or serological changes, and there was no direct or indirect evidence of the agent. In contrast, all the calves in group 3 had mild pyrexia and seroconverted on day 8, and in one of them E phagocytophila organisms were visible in leucocytes, and the PCR on the buffy coat was positive on day 8.