Cytotoxic lymphocytes, including natural killer cells, lymphokine-activated killer cells, and cytotoxic T lymphocytes, adhere to and lyse cancer cells by recognizing cell surface antigens. Among the cell surface antigens, intercellular adhesion molecule-1 (ICAM-1) and HLA class I antigen are important for the cytotoxic activity of lymphocytes. The ICAM-1 and HLA class I antigen were examined in gastric cancer cell lines MKN-28 and MKN-45 by flow cytometry to determine whether their expression on the cell surface is enhanced by interferon gamma (IFN-gamma). The cell expression rate [stained cells/10(4) cells x 100(%)] was only 10% in ICAM-1 and about 20% in HLA class I antigen without IFN-gamma, but reached 70% in ICAM-1 and up to 60% in HLA class I antigen after incubation with IFN-gamma for 24-96 h. This enhanced expression of cell surface ICAM-1 and HLA class I antigen by IFN-gamma might increase sensitivity for cytotoxic lymphocytes.