We have determined the tentative amino acid sequence of the beta chain (M(r) 20,000) of the lectin favin. In previous studies, we have shown that the alpha chain (M(r) 5600) of this lectin is homologous to a region in the middle of the concanavalin A (Con A) sequence (residues 70-119). Now we present evidence that the beta chain is homologous to two discrete segments of Con A. The homology begins at residue 120 of Con A, extends to the COOH terminus (residue 237) and continues without interruption through the NH(2)-terminal 69 residues of Con A. Together, the alpha and beta chains of favin account for a polypeptide chain equivalent in size to that of Con A. The comparison of the two proteins thus reveals a circular permutation of extensive homologous sequences. The favin molecule contains residues identical to many of the residues postulated to be involved in sugar binding by Con A, and contains all of the direct metal ligands as well as residues homologous to most of the residues that form the beta-pleated sheets of Con A. These homologies suggest that the three-dimensional structures of the two lectins are likely to be very similar. Moreover, favin appears to be even more closely related in primary structure and sugar specificity to the lectins from pea and lentil, raising the possibility that all of these lectins may have structures that resemble Con A. Some of these similarities may also extend to the lectins from soybean, peanut, and red kidney bean, which have different sugar specificities but share sequence homologies with the favin beta chain.