An 8-week feeding trial was conducted to determine the effect of dietary glutamine (Gln) on the erythrocyte function in juvenile Jian carp. The results indicated that dietary Gln caused significant increases in haematocrit, erythrocyte count, haemoglobin concentration, mean corpuscular volume and mean corpuscular haemoglobin content in carp blood. Dietary Gln caused significant decreases in the levels of superoxide anion, hydrogen peroxide, met-haemoglobin, malonaldehyde and protein carbonyl and increases in the level of reduced glutathione and the activities of Na+, K+-ATPase and lactate dehydrogenase in carp erythrocytes. These results demonstrated that dietary Gln could improve the function of erythrocytes in fish. Meanwhile, the present study explored the effect of dietary Gln on hydroxyl radical (center dot OH)-induced apoptosis in carp erythrocytes. We found that dietary Gln prevented the center dot OH-induced apoptosis by inhibiting the generation of reactive oxygen species and the oxidation of cellular components, and restoring the non-enzymatic and enzymatic antioxidant activities in fish erythrocytes. These results revealed that the effects of Gln on the function may be closely associated with apoptosis in fish erythrocytes. Moreover, our data suggested that dietary Gin raised the levels of alanine (Ala), citrulline (Cit) and proline (Pro) in plasma of carp. We then examined the protective effects of Ala, Cit and Pro at the physiological concentrations in center dot OH-induced carp erythrocytes. The results indicated that Ala, Cit or Pro at the physiological concentrations suppressed apoptosis and the oxidation of cellular components, and restored the GSH antioxidant system in the center dot OH-induced carp erythrocytes. Moreover, the combination of Ala, Cit and Pro at the physiological concentrations produces a greater protective effect than their individual effects in fish erythrocytes. These results displayed that the effects of Gln on erythrocytes are at least partly dependent on that of its metabolites in fish. Ala, Cit, Pro and their combination could serve as an antioxidant or apoptosis-inhibitor in fish.