In recent years,the immune-modulatory role of all-trans astaxanthin from different pigment sources has been studied.It was reported that all-trans astaxanthin might exist as three stereoisomers,and the composition of all-trans stereoisomers in natural materials differs from that of synthetic products.However,the different biological effects of various all-trans stereoisomers still remain unclear.In the present study,we evaluated the bioactivity of three astaxanthin stereoisomers,(3S,3’S)-trans-,(3R,3’R)-transand meso-trans-astaxanthin,in regulating cell-mediated immune response using mice lymphocytes and peritoneal exudates cells(PECs) systems.After the treatment with three astaxanthin stereoisomers(20 μmol L-1),the lymphocyte proliferation capacity,neutral red phagocytosis of PECs and natural killer(NK) cell cytotoxic activity were comparatively assessed.The results showed that all three astaxanthin stereoisomers significantly promoted lymphocyte proliferation,phagocytic capacity of PECs,and cytotoxic activity of NK cells.Moreover,the(3S,3’S)-trans-astaxanthin exhibited a much higher response than others. In recent years, the immune-modulatory role of all-trans astaxanthin from different pigment sources has been studied. It was reported that all-trans astaxanthin might exist as three stereoisomers, and the composition of all-trans stereoisomers in natural materials differs from that that of synthetic products. Yet, the different biological effects of various all-trans stereoisomers still remain unclear. the present study, we evaluate the bioactivity of three astaxanthin stereoisomers, (3S, 3’S) -trans-, (3R, 3’R) -transand meso-trans-astaxanthin, in regulating cell-mediated immune response using mice lymphocytes and peritoneal exudates cells (PECs) systems. After the treatment with three astaxanthin stereoisomers (20 μmol L -1), the lymphocyte proliferation capacity, neutral red phagocytosis of PECs and natural killer (NK) cell cytotoxic activity were comparatively assessed. The results showed that all three astaxanthin stereoisomers significantly promoted lymphocyte proliferation, phagocytic capacity of PECs, and cytotoxic activity of NK cells. Moreover, the (3S, 3’S) -trans-astaxanthin exhibited a much higher response than others.