In the Part I, an apparent life-prolonging effect was observed in mice with transplanted tumors when OK-432 was given i.p. or s.c. as a pretreatment, indicating the presence of the host-mediated action. In this paper, the peritoneal cells were used to investigate the mechanism of its host-mediated antitumor action. Its results are described below: 1) When OK-432 was injected into the peritoneal cavity of mice, peritoneal cells, particularly macrophages, were observed in large quantities. 2) Thus far obtained, these peritoneal macrophages demonstrated an antitumor activity. 3) These exudated macrophages were found to enclose a tumor cell in both mice and patients with carcinomatous peritonitis or pleuritis, when OK-432 was injected into the peritoneal or pleural cavity. 4) There is a tendency to protect mice against tumor rechallenge when mice are treated i.p. or s.c. with OK-432 after pre-immunization of mice with the irradiated tumor cells. Therefore, the effect of OK-432 for tumor specific immunization was suspected, experimently. The results stated above lead to the conclusion that the host-mediated antitumor action of OK-432 is probably due to the activated local macrophages and the increased host immune reponse to the tumor specific antigen.