We estimated the nuclear DNA content of unfixed erythrocytes of the grass carpCtenopharyngodon idellaby fluorescence flow cytometry using propidium iodide as a fluorochrome. Frozen erythrocytes from the domestic chickenGallus galluswere thawed and used as an internal reference for simultaneous analysis with grass carp samples. The nuclear DNA content of erythrocytes from diploid grass carp (mean ± SD) was 2.00 ± 0.01 pg/cell, whereas triploids possessed 2.97 ± 0.03 pg/cell. Variations in the nuclear DNA content ofdifferent chickens were standardized in relation to the DNA content of fresh human leukocytes (7.00 pg/cell). The value for DNA content of the chicken erythrocytes (around 2.5 pg/cell) occupied a position intermediate between the two grass carp values, and thus provided a clear reference for discrimination between diploids and triploids.