The seasonal variation of laminaran content in Sargassum hornori collected monthly from October. 1974 to March, 1975. The investigation reveals that thereis no laminaran in October. The content gradually increased from November until itreached a maximum of 4.32% in March of the next year. Dried seaweeds are extracted with a dilute HCl solution. The laminaran was isol-ated and purified by the mothod of frantional precipitation with alcohol. Laminaran was sulfated in formamide with chlorosulphonic acid. The optimumsulphating temperature and treating times were found to be at 35℃ and for 45 min.The resultant Na laminaran sulfate contains about 20% S.