High mycoplasmal infection ratio in gastric cancer tissues suggests a possible association between my-coplasma infection and tumorigenesis. Because TNF-a plays an important role in carcinogenesis caused by microbes in-fection and P37 is a major immunogen of mycoplasma hy-orhinis (M. hyor.), investigating whether P37 could induceexpression and secretion of TNF-a will be very significant to elucidate the possible molecular mechanism of gastric car-cinogenesis involved with M. hyor. At the present study, we cloned full gene of p37 by PCR and mutated the 7 codes of TGA into TGG firstly, then expressed the P37 protein suc-cessfully with pGEX-4T-1 vector in E. coli, which was veri-fied with Western blot. By RT-PCR and sensitive L929 cell toxic assay, we found that P37 protein could induce expres-sion and secretion of TNF-a from human peripheral bloodmononuclear cells, and the inducing activity of P37 could be dramatically blocked by McAb PD4. These results suggestthat the induction of TNF-a secretion by P37 probably plays an important role in diseases caused by M. hyor. infectionand needs to be further investigated. High mycoplasmal infection ratio in gastric cancer tissues suggests a possible association between my-coplasma infection and tumorigenesis. (M TNF-a plays an important role in carcinogenesis caused by microbes in-fection and P37 is a major immunogen of mycoplasma hy-orhinis. hyor.), investigating whether P37 could induce expression and secretion of TNF-a will be very significant to elucidate the possible molecular mechanism of gastric car-cinogenesis involved with M. hyor. At the present study, we cloned full gene of p37 by PCR and mutated the 7 codes of TGA into TGG first, then expressed the P37 protein suc-cessfully with pGEX-4T-1 vector in E. coli, which was veri-fied with Western blot. By RT-PCR and sensitive L929 cell toxic assay, we found that P37 protein could induce expres-sion and secretion of TNF-a from human peripheral blood mononuclear cells, and the inducing activity of P37 was able to be blocked blocked by McAb PD4. These results suggest that the induction of TNF-a secretion by P37 probably plays an important role in diseases caused by M. hyor. infectionand needs to be further investigated.