Abstract: The proliferation of erythroid cells in peripheral blood was detected by anti-PCNA monoclonal antibody (PC10) with a means of immunocytochemistry in diploid and triploid rainbow trout. The rate of PCNA positive erythroid cells in triploidy peripheral blood was detected by flow cytometry. The erythrocytes which were positive for PCNA were confined in the cells with high nuclear-cytoplasmic ratio. About 6.4% erythrocytes were found positive for PCNA in triploid rainbow trout while only 0.4% in diploids. And the PCNA positive rate of the erythrocytes was 0.37% in diploids and 8.39% in triploids in average. PCNA, a proliferating marker is found to be expressed only in immature cells in the peripheral erythrocytes of triploid rainbow trout. This suggests that peripheral erythrocytes with the ability of proliferating are immature cells.