Localization of MG_7 antigen was investigated in situ by immunohistochemic and immunoclectron microscopic technique in 62 cases of gastrointestinal carcinoma (GICA). The detective rate of MG_7 antigen was 80. 77% in gastric carcinoma (21/26 cases) and 94.44% in colorectal carcinoma (34/36 cases). Its expressing intensity and distribution were associatcd with the differentiation of tumor cells. Most of the poorly differentiated tumors showed much stronger MG_7 antigen expression and cytoplasmic staining, while most of the differentiated tumors showed membranic or mixed staining, mainly distributed in lumen side. Immunoelectronimicroscopic study revealed that MG_7 antigen predominantly existed in lumen margin microvilli, cell membrane and cytoplasm bencath the cell membrane in well differentiated tumor cells, existed in secretory granules, Golgicomplex and endoplasmic reticulum in poorly differetiatd tumor cells. The finding suggest that MG_7 antigen be a kind of secretory substance syntheszed by GICA cells. The mutual relationship between MG_7 antigen localization and tumor cell differetiation as well as its clinical significance were also discussed.