The expression of proliferation-associated nuclear antigen p105 of gastric carcinomas was studied by multiparameter flow cytometric and immunohistochemical technique. Multiparameter flow cytometric analysis revealed that p105 expression was rarely observed in the resting cells of normal gastric epithelium. On the contrary, the immunofluorescence (IF) intensity of cancer cells in G1 phase was approximately two-fold to three-fold greater than that of G0 phase of cancer cells or normal gastric epithelium. The p105 antigen content of cancer cells increased with cell cycle progression, and increased more rapidly in cells in late S-phase than in cells in G0 and early S-phase. Microphotometric study demonstrated that cells in M-phase exhibited a dramatic increase in the amount of the antigen, and the IF intensity of the mitotic cells were approximately five-fold to ten-fold greater than that of cells in G1 and S-phase. These results indicate that the antigen demonstrated by the p105 monoclonal antibody is present in the G1, S, G2, M-phases, but not in the G0 phase. Immunohistochemical technique demonstrated that patients with lymph node metastases are more likely to have high p105-positive rates than did node-negative patients. The mean p105-positive rates of aneuploid tumors was significantly higher than those of diploid tumors. From these results the authors speculate that the measurement of p105-positive rates may be a powerful prognostic indicator of gastric carcinoma.