The human leukemia cell line, HL60 is very sensitive to various apoptotic stimuli and p53-null. The death-related cysteine proteases of the caspases family play a central role in the execution phase of apoptosis, and we recently reported the importance of serine protease activation in camptothecin-induced apoptotic endonuclease activation in HL60 cells. In the present study, we investigated the role of caspases (ICE/CED-3-related cysteine proteases) and serine proteases in cell death induced by the topoisomerase I inhibitor, camptothecin, in HL60 cells and in a cell-free system. We found that CPP32 is activated during camptothecin-induced apoptosis, and that N-benzyloxycarbony-Val-Ala-Asp (O-methyl) -fluoromethyketone (Z-VAD-fmk), a cell permeable caspase inhibitor blocks all features of apoptosis: morphological changes, cleavage of caspase 3 (CPP32/Yama/Apopain) and poly(ADP-ribose) polymerase, lamin B degradation and DNA fragmentation. However, Z-VAD-fmk and two other ICE/CED-3 inhibitors, YVAD-CHO and DEVD-CHO, were inactive in a cell-free system reconstituted from nuclei of untreated HL60 cells and cytosol from camptothecin-treated cells, suggesting that caspases are not required for endonuclease activation or lamin B cleavage in the cell-free system. By contrast, the serine protease inhibitors, 3,4-dichloroisocoumarin (DCI) and L-1-chloro-3-(4-tosylamido)-4-phenyl-2-butanone tosyl-L-phenylalanine chloromethyl ketone (TPCK), abolished the apoptosis-associated biochemical changes induced by camptothecin both in whole cells and in a cell-free system. DCI also inhibited CPP32 cleavage. Taken together, these results suggest that in HL60 cells, both CPP32 and serine proteases are activated in camptothecin-induced apoptosis.