Treatment of female SpragueDawley rats with the potent mammary gland carcinogen 7,12dimethyC benz[a]anthracene (DMBA) results in the formation of DNA adducts with dG and dA and in the induction of 6thioguanineresistant (TG') lymphocyte mutants. In this study, we have examined the types of mutations induced in T G lymphocytes from DMBAtreated rats. DNA from 263 TG' lymphocyte clones was screened for mutations in exons 2 , 3 , and 8 of the hprt gene by polymerase chain reaction (PCR) amplification of the exons followed by heteroduplex analysis using denaturing gradientgel electrophoresis. Twenty-five of the clones produced hetero-duplexes in exon 2, 35 produced heteroduplexes in exon 3 , and 36 produced heteroduplexes in exon 8. Direct sequence analysis of the heteroduplexes revealed 96 mutations, and at least 74 of these mutations were produced independently. Eighty-five of the total mutations were simple base pair (bp) substitutions, with A + T and G + T transversions being the predominant types. Seven mutations were deletions, three were complex bp substitutions, and one was an insertion. The results suggest that the types of mutations produced by DMBA in rat lymphocytes are specific to the DNA adducts produced by this compound. o 1996 WiIey-Liss, Inc.'