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 (ii) Treatment with coenzyme Qio 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.