Montan wax is a mineral wax extracted from lignite type coal. It has been registered as a food additive in Japan though there have been no reports of toxicological evaluation, mainly due to the fact that it is considered a natural product. As part of a general safety assessment of montan wax, we have performed a 90-day toxicity study in Fisher 344 (F344) rats. Groups of 10 males and 10 females were given the material at dose levels of 0 (Group 1), 0.56 (Group 2), 1.67 (Group 3), or 5% (Group 4) in the diet for 90 days. During the experiment, there were no remarkable changes in general conditions and no deaths occurred in any group. On hematological examination, Hb, Ht, MCV and MCH were significantly decreased and WBC was significantly increased in all treated rats. On serum biochemical examination, AST and ALT were found to be elevated more than four fold in all treated groups as compared to the respective control group values in both sexes. Furthermore, relative organ weights for the liver, spleen, lung and kidneys were increased in all treated groups of both sexes. Histopathological examination revealed diffuse multiple granulomas in the livers with severe hepatocyte damage and lymphocytic infiltration. Granulomatous lesions were also apparent in the mesenteric lymph nodes in all treated males and females. These findings clearly demonstrate that montan wax, at doses of more than 0.56% in the diet, induces multiple granulomas with severe inflammation in the liver. Because pathological, hematological and serum biochemical changes were observed in the lowest dose group, a no-observed-adverse-effect level (NOAEL) could not be determined in the present study.