Members of the coenzyme Q group increase the phagocytic activity in rats, as measured by the carbon clearance technique, and increase the hemolytic antibody formation in mice. In addition, prior treatment with low doses of chloroquine hydrochloride combined with coenzyme Q(10) results in increased numbers of survivors, prolonged survival time, and reduced parasitemia in blood-transferred Plasmodium berghei infection in miceIn an extension of these studies, using emulsions of coenzyme Q(10), I demonstrated the following effects on two tumor systems in mice: (i) Treatment with coenzyme Q(10) decreased splenomegaly and hepatomegaly and increased the number of surviving mice infected with Friend leukemia virus. (ii) Treatment with coenzyme Q(10) reduced the percentage of mice with tumors, increased the number of survivors, and reduced the tumor size in mice with tumors induced by 3,4,9,10-dibenzpyrene. The effect on both tumor systems was dose-dependent. These studies support the hypothesis that the host defense system plays a definitive role in the defense of the host against invasion by various agents, including neoplasia.