Programmed Cell Death (PCD) induced by H2O2 was investigated in Chinese Hamster Ovarian Cells (CHO), and compared to PCD induced by Cisplatin (25-50 μM), which is a well-known apoptosis- inducing agent. In preliminary experiments, CHO cells were treated with low concentrations of H2O2 (250-500 μM) continuously or for 3 hours followed by washing and further incubation in complete growth medium for I- 10 days. Fluorescence microscopy of acridine orangestained cells indicated morphological changes of PCD, including membrane Webbing, nuclear condensation and fragmentation. Similar changes were observed in cisplatin-treated cells. DNA laddering also revealed that DNA fragmentation began to occur in both adherent and non-adherent cells by day 2. To prevent H2O2-mediated PCD, the catalase gene was isolated and ligated into a mammalian expression vector (pCDM8). Transiently transfected cells contained approximately 17-fold increased catalase levels, as measured by ELISA. Overexpression of intracellular catalase provided protection of the CHO cells from H2O2 as measured by chromium-51 release and MTS assays. Furthermore, Cisplatin-mediated PCD was prevented by intracellular catalase Overexpression. The protective effect of catalase Overexpression on PCD induced by other agents is being investigated further.