The localization of three monoclonal antibodies (mAb) against gastric cancer was studied on two human gastric cancer cell lines by immunoelectron microscopic technique. It was shown that the corresponding antigens of mAb 3G9 and 3H11 were distributed on the microvilli (M) and non-microvillous (NM) plasma membrane of target cells, with varying M to NM ratios depending on the mAbs and target cells used. However, the corresponding antigens of mAb, PD4 was only localized on the surface of round or finger-like bulges of target cells and never on the microvilli and non-microvillous plasma membrane. Since the nature and function of these tumor antigens are yet to be identified, the implication of the different distributions of these tumor antigens remains to be claifated.