Two groups of 50 BALB/cj inbred male mice were fed 300 ppm of a polychlorinated biphenyl, Aroclor 1254, in the diet for 11 and 6 months, respectively. The 6 months' feeding was followed by 5 months' recovery. Two additional groups of 50 mice each were fed plain chow. All 22 surviving mice fed Aroclor 1254 for 11 months had greatly enlarged livers representing 25% of their body weight, whereas those fed the experimental diet for 6 months only had slightly, but significantly, enlarged livers. Adenofibrosis was observed in all 22 livers of mice fed Aroclor 1254 for 11 months but not in the other groups. Of the 22 mice fed 300 ppm Aroclor 1254 for 11 months, 9 had 10 hepatomas measuring 0.1-1.5 cm in diameter. One of 24 surviving mice fed Aroclor 1254 for only 6 months, followed by a control diet for 5 months, had a hepatoma 0.3 cm in diameter. No controls had hepatomas.