Male Ah-responsive C57BL/10ScSn mice received a single dose of iron-dextran (600 mg Fe/kg) and were fed a diet containing 0.01% of the PCBs mixture Aroclor 1254 for up to 12 months. Iron caused a marked synergistic increase in liver size from 2 months with greatly elevated mitotic rates, numbers of basophilic foci and incidences of bile duct and oval cell proliferation. Although at 4 months much of the liver enlargement was due to iron-depleted hyperplastic regions and lipid accumulation, by 8 months seven out of nine mice had nodules (hepatocellular adenomas) whereas none were observed in the Aroclor alone group. After 12 months, 16 out of 18 mice had multiple nodules and/or hepatocellular carcinomas whereas only one of 16 mice was positive in a group not given iron. Basophilic nodules were more common than clear cell nodules in those mice with carcinomas than in those animals without. Preloading with iron also greatly enhanced the development of cholangiofibrosis at 8 and 12 months. Preliminary experiments with the polybrominated biphenyl mixture Firemaster BP-6 indicated a similar synergistic interaction with iron. No effects of iron and Aroclor 1254 on the liver were observed in the Ah-nonresponsive strain DBA/2. Iron potentiated the development of uroporphyria after exposure to those chemicals in C57BL/10ScSn mice but not in the DBA/2 mice. Therefore in C57BL/10ScSn mice the carcinogenicity of PCBs and possibly PBBs, is modulated by iron status and probably not at a late stage where these chemicals may act in a promotional manner.