An agglutinating antigen and a rapid card test (CT) for equine piroplasmosis was developed. The antigen for the CT was prepared from lyophilized Babesia caballi complement-fixation (CF) antigen. Serum and plasma samples for testing were obtained from known B caballi-infected horses and clinically normal horses maintained at the laboratory. Serum samples also were obtained from horses outside the continental United States, in areas where piroplasmosis is endemic. Comparative CT and CF tests were done on all samples. The CT correctly identified 85% of 192 plasma samples from known infected and normal horses and 92% of 188 serum samples from these same horses. The CT results agreed closely with CF results. There was good agreement between CT and CF results. There was good agreement between CT and CF results on serum samples from horses outside the United States. Of 19 CF-positive samples, 90% were also CT-positive and 92% of 177 CF-negative samples were also CT-negative.