Thiamin-binding proteins (TBPs) were widely distributed in plants as well as in microorganisms and animals. In plants, the proteins mainly existed in seeds and were concentrated in the germ. The TBPs isolated from rice, buckwheat, and sesame seeds consisted of much glutamic acid (or glutamin) and bound to thiamin specifically. Sesame seed has three kinds of the TBP whose molecular mass and the optimum pH for thiamin binding are different from the TBPs from rice and buckwheat seeds. The TBPs from these three seeds did not show immunological similarity each other, whereas the TBPs similar to that from rice seed were distributed in gramineous seeds, indicating that the origin of the TBPs might differ. The TBP accumulated during seed development with thiamin and degraded at the early stage of germination accompanying the release of thiamin. These results suggest that TBPs in plant have dual functions as thiamin retainer and storage protein. Thiamin seems to be stored by binding to the protein in dormant seeds, and to be released with the protein degradation and used as a coenzyme when seeds germinate, while TBPs may provide nitrogen at germination.