Calpain inhibitors block while cathepsin inhibitor I (CI) amplifies drug-induced apoptosis in A-549, K-562, and U-937 cells. K-562, U-937, and A-549 cells were treated with 20 μM cisplatin alone or in the presence of 20 μM calpeptin (calpain inhibitor) or 40 μM CI or 20 μM ALLM for 36 hours. Control cells were left untreated or treated with either the same dose of inhibitor only or a corresponding volume of DMSO. (A) Proteins were extracted and subjected to SDS-PAGE and Western blotting with human-specific anti-Bax, anti-Bcl-2, and anti-Bid antibodies. (B) Cell viabilities were measured by the trypan blue exclusion method. Caspase-3 activity was measured as described in “Materials and methods” at 24 hours following the treatments to confirm the viability results. Error bars indicate means ± SD (n = 3).