The Tn syndrome is an acquired form of persistent mixed-field polyagglutination displaying two distinct populations of Tn positive (Tn) and Tn negative (tn) red blood cells (RBCs). We investigated the electrophoretic behavior of RBCs showing polyagglutination from a patient with Tn syndrome by the doppler electrophoretic light scattering (D.E.L.S.) analysis. The mean of zeta potential of normal RBCs from ten individuals was -13.07 +/- 0.61 mV (mean +/- S.D.). The content of membrane-associated sialic acid equated with the zeta potential of RBCs. Among the proteases ficin was most effective on the zeta potential of RBCs. The zeta potential of the patient Tn RBCs and tn RBCs were -4.73 mV and -13.32 mV, respectively. Tn RBCs reduced 64.5% of zeta potential compared with tn RBCs and formed 48.8%. These results may provide some useful information for classification of Tn syndrome.