Effect of the additional treatment with coenzyme Q10 on immunorestoration with Mycobacterium bovis BCG in tumor-bearing mice was investigated. Cell-mediated cytotoxicity in tumor-bearing mice against alloantigenic tumor cells was determined by 51Cr release assay using spleen cells of C57BL/6N mice which had been inoculated subcutaneously with syngeneic melanoma-B16 and immunized intraperitoneally with alloantigenic mastocytoma P815-X2 cells. The cell-mediated cytotoxicity against mastocytoma P815-X2 cells was gradually depressed with the growth of melanoma-B16. The depressed, cell-mediated cytotoxicity in tumor-bearing mice recovered slightly by the treatment with BCG. The recovery effect of BCG on the depressed, cell-mediated cytotoxicity was significantly enhanced by the additional treatment with coenzyme Q10. Coenzyme Q10 did not have an apparent effect on the depressed, cell-mediated cytotoxicity in tumor-bearing mice. These results show that coenzyme Q10 enhances the immunorestoration with BCG in tumor-bearing mice.