Anthocyanins are thought to have antioxidant effects in the body. The effects of two anthocyanidins, malvidin and delphinidin, on the activities of antioxidant enzymes, glutathione reductase (GR), glutathione peroxidase (GPx) and glutathione-S-transferase (GST), and other markers of oxidative stress were examined in HepG2 human hepatocellular carcinoma cells. In study one, cells were treated with anthocyanidins at concentrations of 0, 5 or 10 mol/L for 24 h and 0 or 200 mol/L tert-butyl hydroperoxide (t-BOOH) for 2 h. Study two used a combined dose of 10 mol/L anthocyanidins and 0 or 200 mol/L t-BOOH. Cell proliferation and GR activity were significantly increased by delphinidin; with 5 mol/L having the greatest effect in oxidatively stressed cells. Malvidin also increased cell proliferation, but to a lesser extent than delphinidin. No effect of treatment was observed on GST and GPx activity. Finally, it appeared that combining these anthocyanidins had an antagonistic effect.