To explore the role and significance of costimulatory molecules B7H1,B7H2 and ICOS within tissues of human gastric carcinoma and the possible mechanisms in tumor escape.mRNA expressions of costimulatory molecules including B7H1,B7H2,ICOS and B7-1 in tissues of human gastric carcinoma were investigated by in situ hybridization using digoxigenin-labeled oligonucleotide-probes. The tissue of chronic gastric ulcer was used as a control. All data were analyzed by SPSS statistic software.At the site of gastric carcinoma, mRNA expression levels of B7H1, B7H2 and ICOS were much higher than that of B7-1. Their mRNA positive expression indexes were 0.512+/-0.333, 0.812+/-0.454, 0.702+/-0.359 and 0.293+/-0.253, respectively. The positively stained cells were mainly tumor infiltrating lymphocytes (TILs), and some tumor cells. The difference between them was greatly significant P<0.005. The mRNA expression levels of four molecules were not correlated to the pathological grade and matastasis of gastric carcinoma.ICOS-B7H costimulatory pathway may be predominant at the site of gastric carcinoma. B7-1mRNA might be the basis of ICOS-B7H interaction. ICOS-B7H interaction induces the production of IL-10 which inhibits the antitumor immune responses. Therefore, it is supposed that ICOS-B7H costimulatory pathway may be involved in the negative regulation of cell-mediated immune responses.