Dietary long-chain n-3 PUFA (n-3 LCPUFA) are thought to have immune-modulating effects, but the specific effects and mechanisms are not fully elucidated. The aim of this study was to determine whether dietary n-3 LCPUFA could affect ex vivo oxidative burst in healthy young men. The study had a randomised 2 x 2-factorial design in which subjects were randomly assigned to 8-week supplementation with capsules containing fish oil (about 2.9 g n-3 LCPUFA/d) or olive oil (control). Subjects were also randomly assigned to household use of oils and fat spreads with a high or a low 18:2n-6 content. At baseline and at the end of the intervention, the fatty acid composition of peripheral blood mononuclear cells (PBMC) was analysed by GLC and oxidative burst was studied in whole blood stimulated with zymosan using luminol-enhanced chemiluminescence. The PBMC content of n-3 LCPUFA was markedly increased by the fish oil-supplementation (P < 0.001, compared to the olive oil groups). No effect of the intervention was observed on neutrophil count, but one measure of the zymosan-induced oxidative burst was higher in the fish oil groups (P = 0.03) compared to the olive oil groups. The fat intervention did not in itself affect oxidative burst neither did it change the effect of the fish-oil intervention. The measures of oxidative burst at the end of the intervention period were found to be associated with the DHA content of PBMC (r 0.44, P = 0.016), suggesting a dose-response relationship. These results indicate that n-3 LCPUFA may have immuno-stimulating effects.