The primary objective of this study was to test the hypothesis that inhibition of mitochondrial permeability transition andlor inhibition of caspase family enzymes can block chondrocyte apoptosis induced by H202. Primary human chondrocytes were isolated from normal cartilage by enzymatic digestion. Apoptosis was induced by exposure to H202. Chondrocyte apoptosis was quantified using an ELISA for nucleosome formation. Independent confirmation of apoptosis was obtained by TUNEL analysis. H202 induced apoptosis in primary human chondrocytes in a time and dose dependent manner. The effects of candidate apoptosis inhibitors were then tested. Chondrocytes were pretreated with inhibitors of mitochondrial permeability transition, or one of three different caspase inhibitors, and then incubated with H202. Apoptosis was then measured after 16 h of exposure to H202. Pretreatment with inhibitors of mitochondrial permeability transition did not block apoptosis induced by H202. A non-selective caspase inhibitor, a caspase 3-selective inhibitor, and a caspase 1-selective inhibitor, all blocked chondrocyte apoptosis induced by H202. These results show that H 2 0 2 triggers chondrocyte apoptosis through caspase activation, independent of mitochondrial membrane permeability transition.