Cinnamomin is a novel type II ribosome-inactivating protein (RIP) isolated in our laboratory from the seed of the camphor tree (Cinnamomum camphora). In this paper the physiological role it plays in the plant cell was studied. Northern and Western blotting revealed that cinnamomin was expressed specifically in cotyledons. It accumulated in large amounts simultaneously with other proteins at the post-stages of seed development. Cinnamomin degraded rapidly during the early stages of seed germination. Endopeptidase was proved to play an important role in the degradation of cinnamomin. Western blotting of total proteins from the protein body with antibodies against cinnamomin demonstrated that it only existed in this specific cellular organelle as a storage protein. The similar properties of cinnamomin and other seed storage proteins of dicotyledons were compared. We conclude that cinnamomin is a special storage protein in the seed of C. camphora.