Antisera for the Hy, Gy^a, Jo^a, and Do^a antigens were incubated with cells transfected with pDI2E and then secondarily stained (“Materials and methods”). Sera from donors without detectable anti-Dombrock agglutination activity were used as negative controls (Control). The serum assay was scored positive if the mean fluorescence of the pDI2E-transfected cells fluoresced at levels greater than 2 SDs above the controls. All tests were performed in duplicate or triplicate.