When a strong oxidizing agent, like a drinking water disinfectant, is ingested, it dissolves in the highly organic solutions of the gastrointestinal tract, including saliva and stomach fluid, and comes in contact with tissues. Because it is a strong oxidant, it can be expected to react with organic biomolecules present. Since aqueous chlorine and, to an ever increasing extent, inorganic monochloramine, are widely used disinfectants, it is important to determine what reactions these oxidants can undergo in the body and whether their products exhibit adverse health effects or can be detoxified. The reactions of aqueous chlorine and monochloramine with biomolecules and in biological systems will be reviewed with the intent of assessing their toxicological significance.