The effects of Co2+ (as CoCl2) on the cytotoxicity of green tea polyphenol (GTP) and black tea polyphenol (BTP) extracts towards proliferation of immortalized human gingival epithelial-like S-G cells were studied. The 24 h potencies of GTP and BTP extracts, as determined with the neutral red (NR) cell viability assay, were greatly reduced in the presence of 250, but not of 50, microM Co2+. The cytotoxicities of the GTP and BTP extracts were due, in part, to their generation of hydrogen peroxide (H2O2) in the cell culture medium (DMEM). Progressively increasing the concentration of Co2+ in the tea polyphenol-amended cell culture medium resulted in a lowering of the level of H2O2. The cytotoxicity of freshly added H2O2 to S-G cells was abolished in the presence of 250 microM Co2+ and the level of freshly added H2O2 to cell culture medium was progressively lowered as the concentration of Co2+ was increased. Apparently, under the conditions of these studies, the decreases in the cytotoxicity of GTP and BTP extracts in the presence of CoCl2 were due to the rapid catalytic decomposition by Co2+ of the H2O2 generated in the tea polyphenol-amended cell culture medium.