Duplicate lots of 120 rainbow trout (Salmo gairdneri) fingerlings were fed for 1 year semipurified diets containing 6 ppb aflatoxin B1 (AFB1), 100 ppm Aroclor 1254 (a polychlorinated biphenyl), and 6 ppb AFB1 plus 100 ppm Aroclor 1254. Appropriate controls were also maintained. Samples were taken at 1, 2, 4, 6, 9, and 12 months to monitor tumor incidence, Aroclor 1254 accumulation, and histopathology of liver, spleen, and kidney tissues. At the end of the year, 26 of 37 (70.3%) trout fed 6 ppb AFB1 had hepatocellular carcinomas, compared to 14 of 46 (30.4%) trout fed 6 ppb AFB1 plus 100 ppm Aroclor 1254, a highly significant reduction in tumor incidence in the trout on the Aroclor 1254-containing diet. None of the control or Aroclor 1254-fed fish had liver tumors. Levels of Aroclor 1254 increased rapidly during the first 6 months, then plateaued at approximately 80 ppm on a whole-fish basis. AFB1 inhibited growth but Aroclor 1254 did not. Glycogen depletion of hepatocytes and hyperemia, and white pulp depletion of the spleen were the only changes induced by Aroclor 1254.