The tick-borne pathogen m>Babesia microtim> has become recognized as the leading infectious risk associated with blood transfusion in the United States, yet no Food and Drug Administration–licensed screening tests are currently available to mitigate this risk. The aim of this study was to evaluate the performance of an investigational enzyme immunoassay (EIA) for m>B. microtim> as a screening test applied to endemic and nonendemic blood donor populations.