PIP: It is known that most carcinogenic chemicals can be bound irreversibly to proteins or nucleic acids. A study is presented that explored the irreversible binding of 17alpha-ethinyl estradiol to proteins and nucleic acids by the catalytic action of rat liver microsomes and muschroom tyrosinase. The microsomal binding reaction was inhibited by glutathione and cysteine and its derivatives though it was not affected by lysine and amines. Binding reactivity in the tyrosinase system was inhibited by glutathione, cysteine and its derivates, lysine, and amines. Polylysine did not bind the metabolite of ethinyl estradiol, which suggests the NH2 groups do not bind to the intermediate in the microsomal reaction. However, polylysine did bind irreversibly with esthinyl estradiol metabolites in tyrosinase catalysis. The nonenzymatic reaction of 17beta-hydroxy-4,10(1)-estradiene-2,3-dione with cysteine, lysine, and lysine derivates was found to support the thesis that estrogen o-quinones are the intermediates involved in the protein binding of estrogens in tyrosinase catalysis. An irreversible binding of ethinyl estradiol to DNA and RNA occurred with tyrosinase but not with rat liver microsomes. It was concluded that the results of rat liver microsome catalysis make it unlikely that estrogens are chemical carcinogens.