The characterization and purification of the agglutinin in the Aplysia eggs capable of agglutinating vertebrate erythrocytes and marine bacteria were investigated. The agglutinin prepared from A. kurodai was specific to B cells as to human erythrocytes, while that of A. juliana was non-specific. They were independent upon divalent cations for hemagglutinating activity, though slight enhanced activity was observed by addition of Ca²⁺. The activity remained unchanged after the digestion with proteolytic enzymes, but was reduced partially by the treatment with 2-mercaptoethanol or periodate. The hemagglutination was inhibited by D-galacturonic acid, but not by D-glucuronic acid. The agglutinin was purified from A. kurodai eggs by affinity chromatography followed by gel-filtration. The purified preparation reacted with B cells and rabbit blood cells as low as 0.06μg/ml. The molecular weight of egg agglutinin was estimated to be 70, 000 daltons. The ratio of subunit (13, 000) to a native agglutinin was very close to 6.