The light protein components of silk fibroin were analyzed by Western blotting using polyclonal antibodies against two synthetic peptides that are segment of the light (L-) chain [Yamaguchi et al. (1989) J. Mol. Biol. 210, 127-139] and P25 [Chevillard et al. (1986) Sericologia 26, 435-449], respectively, and against the whole L-chain. Both the L-chain and P25 were present with fibroin in the lumen of the posterior silk gland and in the cocoon. The L-chain was identified as the major light component that was disulfide-linked to the fibroin heavy (H-) chain, whereas P25 was a minor component whose association with the H-chain was suggested to be non-covalent.