The physical binding of benzpyrene-H3 with coil form of thymus DNA has been studied. Covalent linkage between the benzpyrene-H3 or the degradation products of benzpyrenc-H3 and DNA was generated by photoradiation at the absorption band of benzpyrene with the "yield" of photoproduct around 50%. This finding has been conclusively supported by experiments with precipitation and extraction, sedimentation, electrophoresis, spectroscopy, enzymic hydrolysis, and chromatography of the hydrolytic products.