Mutations induced in liver cells by the hepatocarcinogen 2-acetylaminofluorene (2-AAF) were characterized after i.p. administration on 4 consecutive days at 100 mg/kg per injection in male B6C3F1 Big Blue transgenic mice that harbored the Escherichia coli lacI reporter gene. Animals were sacrificed at 5, 10 or 60 weeks following the last injection, livers removed and DNA packaged in vitro into bacteriophage lambda particles. The bacteriophage were assayed for lacI function by plating on E. coli in the presence of X-gal. Approximately 3 x 10(5) plaques were assayed per animal. Solvent-treated control mice exhibited a slight increase in mutant frequency over time, from 3.93 x 10(-5) at 5 weeks to 5.02 x 10(-5) at 60 weeks. In contrast, treatment with 2-AAF yielded an approximately 2-fold increase in mutant frequency at 5 and 10 weeks after treatment relative to controls, with frequencies of 8.13 x 10(-5) and 7.43 x 10(-5) respectively. However, by 60 weeks post-treatment the mutant frequency was not significantly increased over concurrent controls. Similar to results in other systems, 2-AAF induced predominantly single base changes targeted to G:C base pairs, primarily G:C-->T:A transversions (27%). In contrast to results in other bacterial and eukaryotic systems, no deletions were observed among the 2-AAF-induced mutations and the 4 base hot spot deletion that is frequently observed in lacI in E. coli was not observed in this system, suggesting that the lacI transgene may be relatively refractory to frameshift mutations in vivo in the mouse.