The preventive effect of purified cell wall of Bifidobacterium infantis, Whole Peptidoglycan (WPG), on the occurrence of mammary tumor induced by 7,12-dimethylbenz(a)anthracene (DMBA) in SD strain female rats was investigated. Rats were given p.o. with 20 mg of DMBA and then received s.c. 200 mu g of WPG weekly for 20 weeks starting on the day of DMBA administration or for 22 weeks, 2 weeks prior to DMBA administration. In the control group, the first tumors appeared 9 weeks after DMBA administration and the cumulative tumor incidence reached 100% within 17 weeks. Subcutaneous injections with WPG weekly for 20 or 22 weeks significantly protected animals from tumor development. The final tumor incidence in the WPG-treated groups was 60-72% at 20 weeks following DMBA administration. On the other hand, WPG was not effective on the delay of the date of first tumor appearance and the growth of tumors once developed. These results suggest that the preventive effect of WPG on the tumor development greatly depends upon the physiological conditions of the hosts in the system of DMBA-SD female rats.

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