Three kinds of morphologically distinct cell wall preparations were isolated from heat-killed Bifidobacterium infantis and examined for the relative antitumor efficacy with syngeneic Meth A fibrosarcoma in BALB/c mice. Ultrastructural examinations revealed that cell wall skeleton (CWS) did not retain morphologically recognizable cell wall structure but showed fibrous structure. By contrast, a new cell wall preparation, whole peptidoglycan (WPG), which was isolated from whole cells without being subjected to physically destructive methods, completely retained the intact cell wall structure. When WPG was disrupted by sonic treatment, it retained some degree of physical integrity of cell wall structure, as compared with CWS. The results of chemical analysis indicated that the three cell wall preparations had similar chemical properties. A single s.c. injection of either CWS, WPG, or sonicated WPG in a mixture with tumor cells resulted in a significant suppression of the tumor growth. They were of equally high activity. However, when WPG, sonicated WPG, or CWS was injected intralesionally five times into mice bearing 5-day-old tumors, the incidence of complete tumor regression was demonstrated to decrease in the order of 70, 40, and 20%, respectively. The in vitro cytotoxicity test excluded the possibility that the tumor cell destruction was the result of direct cytotoxicity of the cell wall preparations. From these findings, it was concluded that WPG was an active stimulator of host-mediated response at the tumor-growing sites.