The antitumor components of the protoplast fusants of Lentinula edodes and Ganoderma lucidum were examined for immunological activity to elucidate the mechanism of their antitumor activity. They did not show any direct cytotoxicity against tumor cells. But being examined for immunopotentiation activity, they increased the number of colonies in the bone marrow stem cells to 3.0 times. They also increased the activities of the acid phosphatase in activated macrophages to 2.1 times and the secretion of nitric oxide in RAW 264.7 to 2.2 times, respectively. They activated the components of the alternative complement pathway. In humoral immunity. they increased the activities of the alkaline phosphatase in differentiated B cells to 1.6 times and the number of plaque forming cells to 1.8 times, respectively. In cellular immunity, they restored the depressed response of delayed type hypersensitivity in tumor bearing mice to normal level.