Results of an Amplex® Ultra Red Protein A-magnetic bead capture plus aptamer-biotin-streptavidin-peroxidase fluorescence assay which immobilized IgG antibodies from 50 µg of murine antiserum against R. rickettsii or R. typhi. The top five aptamer candidates from ELASA screening against each type of antiserum bound with the greatest affinity to antibodies within their cognate antiserum (blue columns) versus antibodies from the other antiserum (red columns). Background fluorescence for deletion controls without aptamers averaged ~ 150 relative fluorescence units (data not shown).