Blood samples from 16 anaplasmosis complement-fixation (CF) positive sheep from a flock near Cambridge, Idaho, were inoculated into 3 intact sheep and 3 splenectomized calves. The 3 sheep became CF positive, and 2 developed parasitemia. None of the 3 calves had signs of anaplasmosis, but all became ill when they were subsequently challenge exposed with blood from known carriers of Anaplasma marginale. After a 75-day observation period, blood from the recipient intact sheep was subinoculated into 3 splenectomized sheep. These splenectomized sheep developed clinical anaplasmosis and became CF positive. In only 1 sheep was there a positive rapid card agglutination test. Therefore, the rapid card agglutination test was not considered to be diagnostic for anaplasmosis carrier status in sheep. Electron microscopic studies were performed on infected sheep erythrocytes to determine the morphologic structure of the organism involved. The causative organism was determined to be Anaplasma ovis.