Tritiated 3-methylcholanthrene (MC) was traced by electron microscope autoradiography in Balb/c mouse epidermis 24 h after topical application in vivo. Quantification ofthe autoradiographs indicated that the distribution of radioactivity was non-random. The frequency of silver grains was similar in both nuclei and cytoplasm of epidermal cells. A concentration of labelling was detected in association with nuclear and cell membranes but to a greater degree with the dermo-epidermal junction. Results are consistent with previous work on the binding of hydrocarbons to soluble proteins and nucleic acids. The significance of the association of radioactivity with the dermo-epidermal junction is discussed.