Gallic acid (3,4,5-trihydroxybenzoic acid, GA) is known to induce apoptosis in cancer cells at lower IC₅₀ values compared with values for normal cells. Apoptosis is inhibited completely by the addition of conditioned medium from cultured hepatocytes, whereas it is not prevented by conditioned media from tumor cells. We therefore studied the reason for the different response to GA-induced apoposis. GA-induced dRLh-84 cell death was completely abolished by the addition of peroxisome or cytosol as well as conditioned medium from primary cultured rat hepatocyte. As GA-induced cell death is known to be mediated by reactive oxygen species (ROS) and intracellular Ca²⁺, we determined the type of ROS generated by GA and found that GA generated hydrogen peroxide in culture medium. The addition of hydrogen peroxide generated by GA induced cell death in dRLh-84 cells. These results suggest that GA-induced cell death is mediated by hydrogen peroxide. On the other hand, the inhibitory activity of hepatocyte medium on GA-induced cell death was completely abolished by anti-catalase antibody. When the amount of catalase antigen was determined by Western blotting analysis, conditioned medium and the cytoplasm of hepatocytes contained high concentrations of catalase. Conditioned media from various tumor cell lines did not contain catalase, and the cytoplasm contained only low levels of catalase. These results show that GA-sensitive cells, including various tumor cells, produce only small amounts of catalase and secreted little enzyme into media, suggesting a lack of protective machinery against GA. In contrast, GA-insensitive cells, including hepatocytes, produce large amounts of catalase and release it in medium, resulting in the development of insensitivity to GA. In conclusion, catalase contents in cells determine different sensitivity to GA.