Sporamin accounts for about 60% to 80% of total soluble protein in sweet potato tubers, and the predicted protein sequence of sporamin shares significant amino acid sequence identity with some Kunitz-type trypsin inhibitors. We constructed three recombinant plasmids with cDNAs that encode preprosporamin, prosporamin, and sporamin, and these three were expressed in Escherichia coli cells as fusion proteins. All three forms of sporamin expressed in E. coli were shown to have strong inhibitory activity to trypsin in vitro, suggesting that post-translational modifications are not essential for trypsin inhibitory activity. Northern blot analysis showed that sporamin transcripts could be systemically induced in leaf tissue of sweet potato by wounding. Therefore, sporamin may have a defense role as a protease inhibitor, in addition to its role as a storage protein.