We investigated effects of astaxanthin on antitumor effector activities of immunocytes including NK cells suppressed by stress in mice in order to define the immunological significance of astaxanthin when combined with restraint stress treatment. When the mice given restraint stress alone, the total number of spleen cells, and NK cell activities per spleen were reduced to the lowest level on day 3, and restraint stress also caused a significant increase in lipid peroxidation of liver tissue. On the other hand, metastatic nodules were observed in the liver of syngenic DBA/2 mice on day 12 after inoculation with mastocytoma P815. The hepatic metastasis was promoted further by restraint stress when it was applied on day 3 before P815 inoculation. When astaxanthin was consecutively administered orally prior to the restraint stress treatment, asthaxanthin greatly reduced both the metastatic nodules and lipid peroxidation, and improved the NK cell activity that should have been reduced by restraint stress. These results suggest that astaxanthin may improve antitumor immune responses via inhibition of lipid peroxidation in the oxidation process caused by stress.