Increasing evidence suggests that lysosomal proteases are actively involved in apoptosis. Using HeLa cells as the model system, we show that selective lysosome disruption with l -leucyl- l -leucine methyl ester results in apoptosis, characterized by translocation of lysosomal proteases into the cytosol and by the cleavage of a proapoptotic Bcl-2-family member Bid. Apoptosis and Bid cleavage, but not translocation of lysosomal proteases to the cytosol, could be prevented by 15 μ m l - trans -epoxysuccinyl(OEt)-Leu-3-methylbutylamide, an inhibitor of papain-like cysteine proteases. Incubation of cells with 15 μ m N -benzoyloxycarbonyl-VAD-fluoromethyl ketone prevented apoptosis but not Bid cleavage, suggesting that cathepsin-mediated apoptosis in this system is caspase-dependent. In vitro experiments performed at neutral pH showed that papain-like cathepsins B, H, L, S, and K cleave Bid predominantly at Arg 65 or Arg 71 . No Bid cleavage was observed with cathepsins C and X or the aspartic protease cathepsin D. Incubation of full-length Bid treated with cathepsins B, H, L, and S resulted in rapid cytochrome c release from isolated mitochondria. Thus, Bid may be an important mediator of apoptosis induced by lysosomal disruption.