Mechanisms of autoxidation of hemoglobin, and its reactions with H2O2, O2-, and oxidizing or reducing xenobiotics are discussed. Reactive intermediates of such reactions can include drug free radicals, H2O2, and O2-, as well as peroxidatively active ferrylhemoglobin and methemoglobin-H2O2. The contributions of these species to hemoglobin denaturation and drug-induced hemolysis, and the actions of various protective agents, are considered.