Mallory bodies (MBs) were induced in hepatic tumors by administration for up to 85 weeks of a diet containing 10 ppm dieldrin to 50 C3H/He and 62 C57BL/6J X C3H/He B6C3F1 male mice. MBs were seen in 15 of 28 (54%) mice which developed benign hepatic tumors and 33 of 45 (73%) mice with hepatocellular carcinoma, but in only 3 of 39 (8%) mice without hepatic tumors. In mice with tumors, the MBs were predominantly confined to tumor tissue and persisted in a carcinoma transplanted into a nude mouse. MBs were not observed, however, in hepatic tumors of 67 C57BL/ 6J, 49 C3H/He, or 81B6C3F1 mice given 12 jug diethylnitrosamine i.p. on Days 0, 3,9, and 15. Thirty-one of 195 control mice of all three strains had hepatic tumors. Only one of the controls had a tumor with an MB, and no MBs were seen in nontumor-bearing livers of control animals. These observations, coupled with the results of a previous study in which MBs were observed in hepatocytes of dieldrin-treated C57BL/6J mice, indicate that mice treated with dieldrin are a reliable animal model for the study of MBs.