Fucoidan has been shown to enhance immune function. The objective of this study was to examine the in vitro effect of fucoidan on the chamotactic activity of canine peripheral blood polymorphonuclear cells (PMNs). The chemotactic activity of PMNs was evaluated by method of a modified Boyden chamber assay. The amount of interleukin (IL)-8 in the culture supernatants from peripheral blood mononuclear cells (PBMCs) treated with fucoidan was determined by means of ELISA. Fucoidan itself could not have chemoattract effects for PMNs. However, the chemotaxis of PMNs was remarkably enhanced by culture supernatant from PBMCs treated with fucoidan. Similarly, it was also increased by recombinant canine (rc) IL-8. These chemotactic activities of PMNs were inhibited by addition of anti-rcIL-8 polyclonal antibody (pAb). The amount of IL-8 in the culture supernatant from PBMCs was shown to increase upon treatment of fucoidan as compared with that of untreated PBMCs culture supernatant. These results suggest that fucoidan upregulates the chemotaxis of PMNs, which is mainly mediated by IL-8 released from fucoidanstimulated PBMCs.