The potential for allergic contact sensitization to two polyaromatic hydrocarbons, 7,12-dimethylbenz(a)anthracene and benzo(a)pyrene, was investigated in C3H/HeN mice. For each agent, contact hypersensitivity was achieved by applying a 100-μg dose to the shaved abdomen. An ear swelling response was observed following application of 20 μg of the sensitizing dose to the ear dorsum, whereas unsensitized animals or animals sensitized to the alternate agent did not develop ear swelling. Histological sections of DMBA-challenged ears revealed edema with a marked dermal mononuclear infiltrate. Following adoptive transfer of draining lymph node cells of DMBA-sensitized mice to naive syngeneic mice, ear challenge of recipient mice with the same agent resulted in an ear swelling response. This demonstration of in vivo cell-mediated immunological reactivity to polyaromatic hydrocarbon carcinogens indicates that the immune system can interact with and, potentially, modify the course of chemical carcinogens at a time prior to morphological changes or development of tumor-specific transplantation antigens. © 1987, American Association for Cancer Research. All rights reserved.