A lectin was purified from Japanese sea hare Aplysia kurodai by lactosyl-agarose affinity chromatography. The molecular mass of the lectin was determined to be 56 and 32 kDa by SDS-PAGE under non-reducing and reducing conditions, respectively. It was found to agglutinate trypsinized and glutaraldehyde-fixed rabbit and human erythrocytes in the absence of divalent cations. The lectin exhibited stable thermo-tolerance as it retained hemagglutinating activity for 1 h even at 80 degrees C and showed stability at pH 10. By contrast, it was very sensitive at pH less than 5 and in the presence of the sulfhydryl-group preserving reagent, beta-mercaptoethanol. The hemagglutinating activity by the lectin was specifically inhibited by D-galactose, galacturonic acid, methyl-alpha- and methyl-beta-D-galactopyranoside, lactose, melibiose, and asialofetuin. The association rate constant (k(ass)) and dissociation rate constant (k(diss)) were determined for the lectin to be 4.3 x 10(5) M(-1) x sec(-1) and 2.2 x 10(-3) sec(-1), respectively, using a surface plasmon resonance biosensor. The lectin moderately inhibited cell proliferation in the P388 cell line dose dependently. Interestingly, lectin-treated cells did not show a fragmented DNA ladder as is caused by apoptosis, suggesting that the cell proliferation inhibition was caused by another unknown mechanism.