There is considerable current interest in the possible beneficial health effects of quercetin, catechins, epigallocatechins, epigallocatechin gallates, and related phenolic compounds found in teas, wines, and other plant products. As a result, many laboratories are studying the effects of these compounds on cells in culture. The present paper shows that addition of these compounds to commonly used cell culture media leads to generation of substantial amounts of hydrogen peroxide (H₂O₂). Dulbecco's modified Eagle medium gives the highest H₂O₂ level for all the compounds tested, with levels reaching >400 μM within 2 h for addition of 1 mM concentrations of gallic acid, epigallocatechin gallate, and epigallocatechin. Catechin and quercetin produced lower, but still significant, levels of H₂O₂. McCoy's 5A and RPMI 1640 media also promoted H₂O₂ production from the above phenolic compounds. This rapid generation of H₂O₂ could account for some or all of the reported effects of phenolic compounds on cells in culture.