The effects of repeated low-dose-rate, high-dose-rate X-radiation of the head and neck on lingual tumor induction by 7,12-dimethylbenz[a]anthracene (DMBA) in Syrian golden hamsters were studied. Animals received either topical application as 0.5% DMBA in acetone on the lateral middle third of the tongue three times a week for 15 consecutive weeks, 20-R X-radiation exposures of the head and neck once a week for 15 consecutive weeks, or concurrent radiation and DMBA treatments for 15 consecutive weeks. Animals were examined visually at regular intervals, and all were killed 35 weeks after the start of treatments. All tissues were than examined histopathologically. Animals receiving radiation alone had no detectable changes. Animals receiving DMBA plus radiation had an excess of papillomas compared to animals receiving only DMBA (35% vs. 15%). In addition, an excess of nonlingual oral tumors (lip, gingiva, and floor of mouth) was found in DMBA-treated plus radiation-treated animals versus DMBA-treated animals. These results suggest that repeated, localized, low-level X-radiation exposures enhance chemical tumorigenesis in a variety of oral tissues of Syrian golden hamsters.