The effect of LPSw, a lipopolysaccharide from a water extract of wheat flour, on pain response was investigated using an acetic acid-induced writhing test in mice. LPSw inhibited writhing dose-dependently in the range of 10 ng-10 micrograms/mouse i.v. This effect reached its maximum 1.5-3 h after the LPSw inoculation and was detectable even after 8 h. The analgesic effect of LPSw was inhibited by i.v. injection of naloxone and also beta-endorphin was detected in serum and brain tissue following injection of LPSw. Preliminary clinical trials were done in which LPSw was administered percutaneously to relieve the pain of patients with herpes. The results showed that pain was relieved by this application. LPSw may be the best analgesic drug so far known, since it induces the endogenous mediator of analgesia, beta-endorphin.