Fucoidan has shown numerous biological actions; however, the molecular bases of these actions have being issued. We examined the effect of fucoidan on NO production induced by IFN-γ and the molecular mechanisms underlying these effects in two types of cells including glia (C6, BV-2) and macrophages (RAW264.7, peritoneal primary cells). Fucoidan affected IFN-γ-induced NO and/or iNOS expression both in macrophages and glial cells but in a contrast way. Our data showed that in C6 glioma cells both JAK/STAT and p38 signaling positively regulated IFN-γ-induced iNOS, which were inhibited by fucoidan. In contrast, in RAW264.7 cells JAK/STAT is a positive regulator whereas p38 is a negative regulator of NO/iNOS production. In RAW264.7 cells, fucoidan enhanced p38 activation and induced TNF-α production. We also confirmed the dual regulation of p38 in BV-2 microglia and primary peritoneal macrophages. From these results, we suggest that fucoidan affects not only IFN-γ-induced NO/iNOS production differently in brain and peritoneal macrophages due to the different roles of p38 but the effects on TNF-α production in the two cell types. These novel observations including selective and cell-type specific effects of fucoidan on IFN-γ-mediated signaling and iNOS expression raise the possibility that it alters the sensitivity of cells to the p38 activation.