Genotoxic carcinogens form covalent bonds with proteins as well as with DNA. The adducts which result are useful for assessing exposure to the carcinogen, determining inter-individual differences in metabolism and other carcinogen processing, and perhaps in risk assessment. This commentary reviews the development of molecular dosimetry based on protein adducts and describes some of the principles involved. Also described are studies of the binding of bulky lipophilic carcinogens to proteins, which clearly indicate that a high degree of specificity is characteristic of many carcinogen-protein interactions. Studies which have been conducted with human populations are summarized and some proposals for future studies are made.