Transgenic mice carrying an integrated subgenomic human hepatitis B virus (HBV) DNA fragment coding for the viral envelope polypeptides, represent a model for the study of the mechanisms involved in hepatocarcinogenesis. The mice develop a progressive liver injury characterized by inflammation, regenerative hyperplasia and dysplasia terminating in hepatocellular carcinoma (HCC) at around 18-21 months of age. No alterations in specific oncogenes and tumour suppressor genes in the HCC arising in this transgenic model have been observed. However, onset of liver tumours is significantly earlier in mice treated with aflatoxin B1 (AFB1). In order to examine more generally for genetic rearrangements during the natural history of the disease, DNA multilocus fingerprinting was performed using probes recognizing mouse minisatellites. Liver tumour samples from HBV transgenic mice either untreated or treated with AFB1 transplacentally were included in the study. In a total of 28 tumour samples from HBV transgenic mice receiving no carcinogen treatment, using three minisatellite probes, no alterations were detected. The frequency of rearrangements using any one of the three probes is calculated to be below 0.2%. This result demonstrates that genetic instability in minisatellite sequences is not a common event associated with HBV gene expression and liver injury in this model. In 11 liver tumours from mice exposed to AFB1 transplacentally six had minisatellite alterations (band gains and losses) revealed by at least one of the three probes used. The frequency of rearrangements was between 1.1% and 2% depending on the minisatellite probe. These data show that genetic alterations can be induced by transplacental exposure to AFB1 and suggest that genetic instability could be important in hepatocarcinogenesis with combined exposures to AFB1 and HBV.