The primary structure of the alpha subunit from Lens culinaris lectin was determined by analysis of tryptic peptides and was shown to consist of 52 amino acid residues. The molecular weight calculated on the basis of the sequence is 5928. The whole chain is homologous with the region between positions 75 and 121 from concanavalin A. The NH2-terminal sequence of the beta chain, determined by automated Edman degradation, is homologous to another portion of the concanavalin A molecule, between positions 123 and 165. Comparison of the 94 residues from the lentil lectin alpha and beta chains with concanavalin A reveals the existence of 43 identities. Thirty-four other homologies could have arisen, each by a single nucleotide substitution. This extensive homology suggests that the lentil lectin alpha and beta chains may be proteolytic fragments from a single polypeptide chain of the same length as concanavalin A.