The abilities of various peroxidases to catalyse the peroxidase-oxidase oxidation of seven aminothiols were studied. Cysteamine and cysteine esters were found to be peroxidase-oxidase substrates for eosinophil peroxidase and myeloperoxidase, whereas other thiols tested were inactive or poorly active with these peroxidases. With lactoperoxidase and horseradish peroxidase, all the tested thiols were inactive or poorly active as peroxidase-oxidase substrates. These studies suggest that a main reason for thiols being poor peroxidase-oxidase substrates is because these thiols are poor peroxidatic substrates.