Expression of HLA-ABC and HLA-DR was observed in 24 fresh surgical specimens of both gastric cancer and adjacent mucosa with flow cytometry after staining with monoclonal antibodies. In gastric cancer tissues, the HLA-ABC positive cells were 79.9+-5.3% and the HLA-DR positive cells were 75.2+-t5.1%. These ratio of HLA-ABC and HLA-DR positive cells in gas- tric cancer tissues was higher than those in normal adjacent mucosa ( HLA-ABC; 68.0+-6.6 %, HLA-DR; 62.0+-6.2%). In metastatic gastric cancer, HLA-ABC was higher than normal mucosa in most cases. However, HAL-ABC expression in all cases with peritoneal seeding was less than normal mucosa. In stage I gastric cancers, HLA-ABC and HAL-DR positive cells were more than advanced gastric cancers. HLA-ABC positive cells were more com- mon in well and moderately differentiated gastric cancers than in poorly differentiated cancers (85.7+-26.4%: 76.7+-27.1%). The HLA-DR positive cells were less in aneuploid cancers than in diploid cancers (70.2+-28.1%: 87.8+-7.9%). We established method to quantitativ . analysis of HLA expression in tissue. Expression of HLA-ABC & DR were higher in gastric cancer tissues than in normal mucosa. Among the tumors, the HLA-ABC expression was higher in stage I, well differentiated and small cancers, and HLA-DR in diploid and nonmetastatic cancers.