AbstractWe examined the adjuvant activity of theBifidobacterialCell Wall preparation (WPG) forin vivoimmune responses in mice. We studied three classical immune responses, which are thought to be T-cell mediated responses, to evaluate the adjuvant activity of WPG. the delayed type hypersensitivity (DTH) responses of sheep blood red cell (SRBC)-sensitized mice were significantly augmented by WPG, although the enhancement varied with the timing, route and dosage of injection. the adjuvant activity of WPG was also confirmed by using a glutaraldehyde treated-and Concanavalin A associated-tumor vaccine (G-Con A tumor vaccine) system. BALB/c mice sensitized with G-Con A tumor vaccine and WPG improved synergistically in survival time and cure rate compared with those given G-Con A vaccine alone. Spleen cells of Meth A tumor-bearing mice induced antitumor neutralizing activity with the growth of tumor but the activity declined and disappeared at the late stage of tumor growth (over 28 days after tumor transplantation). On the other hand, antitumor neutralizing immunity was prolonged for as long as 33 days in mice inoculated with Meth A tumor and WPG. the requirement of a T-cell subpopulation in the spleen cells of tumor plus WPG treated mice was confirmed using anti-Thy 1.2 antiserum + complement to deplete them.The adjuvant activities of theBifidobacterialcell wall demonstrated by thein vivoimmune responses predict thatBifidobacteriamay play a role as an immunomodulator in human and animal intestines.