The results of complement fixation (CF) test for equine piroplasmosis on sera from horses destined for international movement from Great Britain and Ireland are presented and analysed. No horses born and continuously resident in the British Isles were found carrying CF antibodies to either Babesia equi or B caballi. Positive animals were found to have association with the following countries where known tick vectors occur: Spain, Portugal, Belgium, France, Poland, USSR and Arabian Gulf countries. Data on the persistence of CF antibodies in animals subjected to repeated testing showed that some animals lost their CF titre within a few months of their first test while others retained a titre for periods up to 13 months of B caballi and 28 months for B equi. Some individuals regained titres on subsequent tests after becoming negative and the results suggest that removal from exposure to infection or, even, treatment does not necessarily eliminate a CF titre within a predictable time limit.