We have reported an antitumor aqueous extract from a brown marine alga Sargassum kjellmanianum ("Hahakimoku" in Japanese). Although the extract was effective in the in vivo growth inhibition of the implanted Sarcoma-180 cells, it was not effective against L-1210-bearing mice. In the present study, we attempted to obtain a polysaccharide fraction with antitumor activity against L-1210 leukemia from this alga, on the assumption that the main active substance may be sulfated polysaccharide, especially fucoidan which is mainly composed of L-fucose and ester sulfate. Two kinds of polysaccharide fractions (SKCF and SKCF-F), which contained L-fucose and ester sulfate in the amount of 12.6% and 15.4%, 23.5% and 17.2% respectively, were first prepared starting with extraction with cold-hydrochloric acid, and their antitumor activity was examined. It was found however that they are not effective. Sulfation of SKCF was then carried out. The resulting sulfate (Sulfated SKCF) was observed to contain nearly 50% more ester sulfate than in SKCF and to be effective against L-1210 leukemia showing an ILS value of 26%. Mechanisms of antitumor action of this sulfate were also discussed from the viewpoints of negativity of ester sulfate and of activation of host-mediated immune response as known in antitumor polysaccharide preparations from other sources.