The galactoside-binding B chain of the cytotoxic protein ricin is apparently derived from a conservative exon-sized 40-residue peptide which is repeated four times in the molecule. A very similar peptide can also be seen in the amino acid sequence of the slime mold lectin discoidin I, which itself appears to be the product of a gene duplication. There is presently no chemical or structural evidence concerning the function of this peptide region. Nevertheless, the size of this unit, its prominence in the structure of ricin B chain, and its apparent conservation in carbohydrate-binding proteins from widely divergent organisms suggest that it may represent an extremely ancient galactoside-binding exon unit.