The cytotoxic activity of Maillard reaction products and coffee was studied using the 3-(4,5-dime-thylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) assay and the neutral red uptake (NRU) assay. Equimolar mixtures of sugars and lysine were heated at 120°C and used to stimulate bovine aorta endothelial cells for 24 h. The cytotoxic activity increased with increase in educt concentration and heating time. Mixtures containing ribose were most active, followed by lactose and glucose. Hydrogen peroxide, which was present in the Maillard mixtures in concentrations between 7 and 87 μM, was identified as one of their major cytotoxic components. H2O2-concentrations increased further up to 130 μM under cell culture conditions. Filter coffee, espresso, and green coffee extract reduced cell viability significantly to 10, 19, and 83% of PBS-treated control. The effect was largely attenuated by the addition of catalase. Nil, 33, and 41 μM H2O2 was measured in green coffee extract, filter coffee, and espresso, respectively, increasing to 13, 369, and 333 μM during cell culture conditions. No additional H2O2 formation was detected when coffee was incubated for up to 5 h without further treatment. In conclusion, hydrogen peroxide is a major product in Maillard mixtures and coffee inducing cell death in vitro.