Incubation of [14C]diethylstilbestrol ([14C]DES) with horseradish peroxidase(HRP)/hydrogen peroxide in the presence of various polynucleotides and proteins led to macromolecular binding of radioactivity. Binding to DNA proved stable against ethanol precipitation, but was completely removed when the DNA was subjected to gel electrophoresis, caesium chloride density centrifugation, and mild hydrolysis. In contrast, binding to protein was stable in gel electrophoresis. The extent of binding did not differ significantly between proteins with and without thiol groups. These results imply that the products of peroxidase-mediated oxidation of DES bind to DNA in a strong but non-covalent manner, whereas binding to protein appears to be covalent and does not depend on the presence of thiol groups. The possible nature of the binding species is discussed.