Laminarin polysaccharides (LP1) were prepared from Laminaria japonica, a marine brown alga with potential biological activities, by hot water extraction, ultrafiltration and gel chromatography; the molecular weights of the LP1s were between 5 and 10 kDa. Laminarin oligosaccharides (LO) derived by hydrolyzing LP1 with an endo-beta-(1-->3)-glucanase from Bacillus circulans were mainly di- and penta-oligosaccharides. Treatment of mouse thymocytes with LO or LP1 (1-4 mg ml(-1)) suppressed apoptotic death around 3- or 2-fold and extended cell survival in culture at a rate of about 30 or 20%. A mouse cDNA microarray showing the genes coding for immune response proteins were induced and apoptotic cell death proteins were reduced significantly by LO provided preliminary information regarding the immunomodulatory mechanism of LO. These results suggest that laminarin oligosaccharides and polysaccharides can be utilized to develop new immunopotentiating substances and functional alternative medicines.