Inhibitors of cysteine proteinases have been suggested to play a role in plant host defense against insect predators. Certain plant viruses, such as potyviruses, employ cysteine proteinases as part of their replication mechanism, suggesting that these inhibitors could be effective antiviral agents as well. In this issue, Gomez-Lim and colleagues demonstrate that tobacco plants engineered to express a rice cysteine proteinase gene are resistant to potyvirus infection (see p. 1223).