Objective B7-H1 is a recently identified costimulatory molecule expressed on APCs and can inhibit the activation and function of T lymphocytes. This research is to study if B7-H1 has a relationship with carcinogenesis and MDR of gastric carcinoma by examining the expressions of B7-H1 in human gastric carcinoma cell lines SGC7901/VCR,SGC7901,BGC823 and in the permanent gastric epithelial cell line GES1. Methods Cells were cultured in RPMI1640 supplemented with 10% fetal calf serum. The B7-H1 mRNA was detected by RT-PCR and the protein of B7-H1 was detected by cytoimmunochemistry staining, cytoimmunofluorescence staining and flow cytometric analysis. The expression levels of B7-H1 mRNA and the protein of B7-H1 in the above mentioned four kinds of cell lines were compared. Results All the four cell lines express B7-H1 mRNA at different levels. And B7-H1 expression level increased in the order of from GES-1 to SGC7901/VCR cell line, BGC-823 and then to SGC7901 cell line. The protein of B7-H1 was found to be distributed in the cell membrane and in a little cytoplasm in all the four cell lines through techniques of cytoimmunochemistry staining and cytoimmunochemistry staining. Further more with the result of flow cytometric analysis, the protein expression level of B7-H1 in all these cell lines was found to be consistent with B7-H1 transcript level expression.Conclusion The results suggest that B7-H1 expression on gastric epithelial cells may promote the growth of gastric carcinoma cells and the expression of MDR gene through inh