In the present study, the effects of trans-MUFA, elaidic acid (EA; 18 : 1-9t) and vaccenic acid (VA; 18 : 1-11t) on rat neutrophil functions were compared with those of cis-monounsaturated oleic acid (OA) (18 : 1-9c) and saturated stearic acid (SA; 18 : 0) (10–150 μm). Trans-fatty acids enhanced neutrophil phagocytic capacity, superoxide (O2∙-) and hydrogen peroxide production, and candidacidal activity. The same effects were observed for OA. Cells treated with trans-MUFA showed reduced production of NO∙, whereas those treated with OA showed an increase in production. Treatment with SA did not provoke significant effect on the parameters investigated. The increase in O2∙- production induced by MUFA was not observed when diphenyleneiodonium, an NADPH oxidase inhibitor, was added to the medium. This finding suggests that MUFA stimulate neutrophil NADPH oxidase activity. The addition of 3-[1-[3-(dimethylamino)propyl]-1H-indol-3-yl]-4-(1H-indol-3-yl)-1H-pyrrole-2,5-dione, a protein kinase C (PKC) inhibitor, and wortmannin, a phosphatidylinositol-3 kinase (PI3K) inhibitor, did not affect O2∙- production induced by MUFA. Therefore, the mechanisms by which MUFA stimulate NADPH oxidase are not dependent on PKC and do not seem to involve PI3K. Experiments using Zn2+, an inhibitor of NADPH oxidase H+ channel, indicated that MUFA activate the NADPH oxidase complex in rat neutrophil due to opening of H+ channel.