In order to investigate the food components having immunopotentiating activity, mice (C3H/He Slc and ddY) were administered with peach juice to induce anti-tumor cytokines including tumor necrosis factor (TNF-α) and interleukin-12 (IL-12) in vivo. Membrane-filtered plum juice that had been adjusted pH and osmotic pressure to the same value as that of physiological saline was injected intravenously (iv) into mice (200 μl/mouse). Three hours later, lipopolysaccharide (LPS, 10 μg/mouse) was administered iv, and another 2 hr later the blood was taken from individual mice. For determination of serum TNF-α, cytotoxic ability of this cytokine against L929 cell line was measured. To quantitate serum IL-12,a sandwich enzyme linked immunosorbent assay (ELISA) was used. Strikingly it was shown that TNF-α activity of the serum taken from plum juice-treated mice increased up to 60 to 80-fold when compared with that of untreated mouse group. As for IL-12,10-fold increase was observed in the plum-injected mice. Both TNF-α- and IL-12-inducing factors containing in the plum juice appear to be heat labile. Furthermore, as the cytokine inducing activity in the plum juice was destroyed by pre-treatment with proteases and periodate oxidation as well, it was suggested that the factors might be a glycoprotein. The chemical structure of the TNF-α- and IL-12-inducing factor(s), however, awaits further investigations.