A method for the purification of anti-H(O) agglutinin of eel serum utilizing preparative Disc electrophoresis apparatus was described. The purified hemagglutinin showed a single band on starch gel electrophoretic analysis. In the analytical ultracentrifuge, the agglutinin had an s⁰_{20, w} value of 6.2 S, and was found to be inhibited weakly by human group OSe saliva. The minimum agglutinating dose against human O group red cells was around 0.2μg/ml.