The development of an enzyme-linked immunosorbent assay (ELISA) for the detection of Babesia ovis antibodies is described. In an initial study, a crude Babesia bovis antigen and a synthetic B. bovis-derived antigen (designated 11C5) were used to screen 46 B. ovis-positive and 55 negative sheep sera. A 95% correlation between the two antigenic preparations was found with the positive sera; no negative sera gave positive reactions. The synthetic antigen was then used in the screening of 1466 sera collected from sheep from 18 regions of Turkey. A high incidence of B. ovis-positive reactions was found from all regions (60-80%) in sheep over 1 year old, while from two smaller samples the incidence in young sheep was much less (28 and 52%). This test is superior to existing ones because the synthetic antigen can be produced in a highly reproducible state, is specific and is stable over extended periods of time.