Objective: We previously reported the immune-enhancing behavior of fucoidan, a sulfated polysaccharide extracted from Gagome kombu (GKF), both in vitro and in animal studies. In the present study, we evaluated the immune efficacy and safety of GKF in healthy Japanese adults.Methods: In this randomized, double blind, placebo-controlled study, 30 subjects who ingested GKF (200 mg/day) or placebo for 4 weeks were enrolled. For evaluation of efficacy, phytohemagglutinin-stimulated cytokine production in whole blood cells was measured. For evaluation of safety, blood chemistry analysis, hematological analysis, and urinalysis were conducted.Results: Almost all cytokine production decreased in samples from the placebo group during the test period. Ingestion of GKF for 4 weeks significantly suppressed the decrease in production of the T helper 1 (Th1)-type cytokines interferon-γ and interleukin-12 as well as the Th1:Th2 ratio. There were no adverse clinical changes in blood analysis and urinary analysis, and no serious symptom was observed.Conclusion: These results indicate that GKF is a useful and safe food ingredient to support immune function.