The effects of high doses of dietary or intraperitoneally (i.p.) injected retinol acetate on the gilthead seabream (Sparus aurata L.) innate immune system were studied. Gilthead seabream specimens were fed a commercial non-supplemented diet containing 1·75mg of vitamin A kg−1 (as control) or the same diet supplemented with 50, 150 or 300mg of retinol acetate kg−1 (as vitamin A source). After 1, 2, 4 or 6 weeks, serum samples and head-kidney leucocytes were obtained from each fish. Serum lysozyme activity and myeloperoxidase (MPO) content were unaffected by the vitamin A diet content. The phagocytic and respiratory burst activities of head-kidney leucocytes were established, as well as their myeloperoxidase content. While phagocytosis was not enhanced by dietary vitamin A intake and was even slightly decreased after 2 weeks, respiratory burst activity was enhanced in specimens fed supplements of 150 and 300mg retinol acetate kg−1 diet for 1 or 2 weeks. Leucocyte MPO content was also enhanced when seabream were fed the highest vitamin A dose for 2 or 4 weeks and after being fed the 150 or 50mg supplemented diets for 4 or 6 weeks, respectively. Three different groups of seabream were i.p. injected with 1ml of phosphate buffer containing an amount of retinol acetate equivalent to the daily dietary supplements from the first experiment (0-control-, 0·05 or 0·30mg 100g−1 biomass). Both injection doses of retinol acetate were toxic for the gilthead seabream which showed hypervitaminic effects.These data show that retinol acetate plays an important role in the gilthead seabream nonspecific cellular immune system due to its antioxidant properties. They also point to the importance of the way in which it is administered, by dietary uptake or intraperitoneal injection.