The processes of nonreversible autooxidation and chemical oxidation of human hemoglobin have been studied. The rate of autooxidation increased in the presence of SH-group containing compounds and Fe2+ ions. Modification of oxyhemoglobin by glutaric and malonic dialdehydes also increased both the rate of autooxidation and oxygen affinity. Modification of SH-groups of Cys-93 beta in hemoglobin increased the rate of autooxidation but decreased the rate of hemoglobin oxidation by potassium ferricyanide. The activation energy of chemical oxidation of hemoglobin at temperatures above 24 degrees C was 39 +/- 5 kJ/mol.