A cup of coffee contains mutagens which produce about 5 X 10(4)-10(5) revertants of Salmonella typhimurium TA 100 without S9 mix. One of the mutagens was identified to be methylglyoxal. Methylglyoxal was present in various beverages such as black tea, whisky, and brandy. Methylglyoxal itself induced tumors in rats when administered by subcutaneous injection. However, the mutagenic properties of coffee were different from those of methylglyoxal. The mutagenicity of coffee was suppressed by catalase, and coffee was found to contain hydrogen peroxide. Furthermore, coffee solution was found to have a hydrogen peroxide-generating system. Instant coffee (15 mg/mL) contains 130 microM hydrogen peroxide immediately after the dissolution of coffee powder in water at room temperature. The concentration of hydrogen peroxide increased with time. The mutagenicity of methylglyoxal was increased by the copresence of hydrogen peroxide. A maximum of 30-fold enhancement was observed. The mutagenicity of black tea but not that of whisky was suppressed by catalase.