Diisopropylfluorophosphate (DFP), a general serine protease inhibitor, inhibited the DNA fragmentation and cell death in MDCK cells treated with ricin, modeccin, Pseudomonas toxin, or diphtheria toxin. A trypsin-like serine protease inhibitor, N-tosyl-L-lysine chloromethyl ketone (TLCK) also prevented ricin-induced DNA fragmentation and cell death, albeit less effectively than DFP. Microscopic observation showed that the morphological changes of MDCK cells induced by ricin were prevented by DFP. DFP did not affect the binding, internalization, or subsequent excretion of ricin, but reduced the degradation of ricin in MDCK cells, suggesting that DFP inhibits at least the cellular protease that may be involved in the degradation of internalized ricin. In addition, SDS-PAGE analysis of cytosolic proteins suggested that DFP-sensitive endogenous proteases are activated in the ricin-treated cells. In the cells treated with DFP, the protein synthesis inhibitory activity of ricin was increased rather than inhibited. The activities of modeccin and Pseudomonas toxin were also slightly increased by DFP, but no effect of DFP on the activity of diphtheria toxin was observed. Therefore, these results suggest that protein toxins have a DFP-sensitive common pathway leading to apoptosis that is distinct from the pathway leading to the inhibition of cellular protein synthesis.