The inhibitory effect of nitric oxide (NO) on the enzymatic activity of CPP32-like proteases in the cell extract from vincristine-treated cells was examined in vitro. NO generated from (+/-)-(E)-methyl-2-[(E)-hydroxyimino]-5-nitro-6-methoxy-S-hexen eamide (NOR1) inhibited CPP32-like protease, which constitute a family of interleukin-1beta-converting enzyme (ICE)-like proteases in a dose-dependent manner. Moreover, recombinant CPP32beta activity was inhibited by NOR1 at same concentration. Inhibition of CPP32-like activity by NO was reversed in the presence of glutathione in the enzymatic reaction mixture. Thus, CPP32-like activity was regulated by NO under redox regulation. These findings suggest that NO may prevent apoptosis by inhibiting the ICE protease cascade under the influence of cellular redox status.