The enhancement of antitumor effects by combination of biological response modifiers (BRM) was investigated on the basis of their action mechanisms. Experimentally, significant inhibition of tumor growth by combination treatment with BRM, which eliminated immune suppressive mechanisms and in turn enhanced immune responses, was observed. Furthermore, inhibition of tumor growth was observed under conditions of uniform biorhythm in mice and the effect of modification of biorhythm was enhanced by combination with BRM. Clinically, combination treatment of plasma exchange and LAK-cell adoptive immunotherapy was discussed.