Chronic irradiation (three times a week) with ultraviolet B light of the skin of hairless mouse Uscd (Hr) strains resulted in the induction of skin tumors after 25 to 41 weeks. Topical applications of 3-aminobenzamide (3AB; 0.1 or 1 m) after each irradiation significantly shortened the earliest time of onset of tumors to 13 to 25 weeks and increased the number of animals that developed tumors over 41 weeks from 67% without 3AB to 73% and 81% with 0.1 and 1 m 3AB, respectively. 3-Aminobenzamide has previously been shown to inhibit radiation-induced transformation in vitro. In vivo, 3AB has the opposite effect, indicating the need for caution in extrapolating from in vitro systems to carcinogenesis in vivo.

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Supported by the Office of Health and Environmental Research, U.S. Department of Energy, Contract DE-AC03-76-SF01012, and by National Cancer Institute Grant CA15605.

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