A new lectin was purified from tubers of Arum maculatum L. by affinity chromatography on immobilized asialofetuin. Although this lectin is also retained on mannose-Sepharose 4B, under the appropriate conditions free mannose is a poor inhibitor of its agglutination activity. Pure preparations of the Arum lectin apparently yielded a single polypeptide band of approximately 12 kD upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. However, N-terminal sequencing of the purified protein combined with molecular cloning of the lectin have shown that the lectin is composed of two different 12-kD lectin subunits that are synthesized on a single large precursor translated from an mRNA of approximately 1400 nucleotides. Lectins with similar properties were also isolated from the Araceae species Colocasia esculenta (L.) Schott, Xanthosoma sagittifolium (L.) Schott, and Dieffenbachia sequina Schott. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration of the different Araceae lectins have shown that they are tetrameric proteins composed of lectin subunits of 12 to 14 kD. Interestingly, these lectins are the most prominent proteins in the tuber tissue. Evidence is presented that a previously described major storage protein of Colocasia tubers corresponds to the lectin.