Isolated gilthead seabream head-kidney leucocytes were incubated in a culture medium supplemented with vitamin C (from 0-control-to 2 mg ml(-1)) or with a combination of different amounts of vitamin C (0.002 to 2 mg ml(-1)) and vitamin E (0.01 to 10 microg ml(-1)) for 6, 24 or 48 h. Afterwards, the cellular ascorbic acid content and the natural cytotoxic activity of leucocytes were evaluated. Leucocyte ascorbic acid levels were enhanced after incubation for 6-24 h with 0-2 mg of vitamin C ml(-1) and above. The natural cytotoxic activity of leucocytes after incubation with vitamin C was also increased for all the assayed concentrations and incubation times except in the case of the highest vitamin C concentration (2 mg ml(-1)) and the longest incubation time (48 h). No statistically significant differences in leucocyte cytotoxic activity were observed when vitamin E was added to the vitamin C, compared with the results of vitamin C alone. For the in vivo study, fish were fed diets supplemented with vitamin C (2.9 g kg(-1) diet) without or with vitamin E (1.2 g of vitamin E kg(-1) diet) for 2, 4 or 6 weeks. Serum lysozyme activity was enhanced to a statistically significant degree when fish were fed with the vitamin C + E supplemented diet for 2 weeks and with the vitamin C and vitamin C + E supplemented diets for 4 weeks. Both groups of fish showed a statistically significant increase in the natural cytotoxic activity of head-kidney leucocytes after 6 weeks of treatment although no differences were observed between treatments incorporating vitamin C alone or vitamin C combined with vitamin E.