The Bowman-Birk protease inhibitor (BBI), a serine-protease inhibitor, has been reported to exert both anticarcinogenic and radioprotective activity. In this work we examined whether this effect is mediated through inhibition of serine-proteases of the trypsin-chymotrypsin type. Using linearized forms of BBI, evidence will be provided that the secondary structure, obligatory for the protease inhibitory function, is not necessary for the radioprotective effect. Detailed analysis indicated that the radioprotective effect is correlated with the chymotrypsin-inhibitory region of the molecule. Using a synthetic nonapeptide lacking protease inhibitor activity, the radioprotective effect of the total BBI molecule could be mimicked, indicating that the radioprotective effect is independent of the protease inhibitor function.