Heat-killed cells of Lactobacillus plantarum L-137 are potent inducers of IL-12 in vitro as well as in vivo and have been shown to have antiallergic, antitumor, and antiviral effects through this induction, which leads to a Th1 type immune response. To determine why L-137 cells induce much greater IL-12 production than the type strain Lactobacillus plantarum JCM1149, we examined the differences in their CW components. The L-137 CW was found to have a higher alanine content and IL-12p40 induction was significantly greater in comparison with JCM1149 CW, whereas peptidoglycans isolated from both strains did not cause IL-12p40 induction. Because in purified CW preparations from gram-positive bacteria, the presence of LTA, the major proinflammatory structure on these bacteria, has been known to have high alanine content, we investigated the responsiveness of both strains to anti-LTA antibody by flow cytometry. L-137 cells reacted more with anti-LTA antibody than did JCM1149 cells. Furthermore, derivative strains of L-137, cured of a specific plasmid pLTK11 of the 15 endogenous plasmids in wild-type L-137, had poor responsiveness to anti-LTA antibody and showed lower IL-12p40 inducing activity than the wild-type L-137 with pLTK11. Our results suggest that LTA expression on the cell surface causes IL-12p40 induction, and that the above internal plasmid of L-137 influences LTA synthesis and expression on the cell surface.