The mutagenicity of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) was investigated in male MutaMouse mice administered 20 mg/kg per o.s. for 4 days and killed 7 days later. Genomic DNA was extracted from liver, kidney and small and large intestine and the mutation frequency (MF) at the lacZ locus was determined using a positive selection assay. Mutant lacZ clones from the intestine were characterized further by direct PCR amplification and DNA sequencing. A total of 57 lacZ mutants from PhIP-treated (40) and untreated (18) mice were analysed. In mutants from the PhIP group, 33% were G:C-->T:A transversions from a total of 65% base substitutions (cf. 17% in the vehicle control group). In untreated control mice, 39% of mutants were G:C-->A:T transitions from a total of 72 % base substitutions (cf. 25 % in the PhIP group). Interestingly, 20% of the PhIP group mutations were due to G:C base pair (-G) deletions (cf. none in controls). This study confirms that PhIP is mutagenic to the intestine of the MutaMouse and induces a spectrum of mutations which are clearly distinct from those spontaneously generated. Also, the PhIP mutation signature in vivo is very similar to that observed for the HPRT and DHFR loci in hamster and human cells in vitro. This suggests that the mutational characteristics of PhIP are well conserved over different reporter genes and between species and that the mutation signature could be of value in molecular epidemiology studies.