This work evaluated the effect of a sulphated fucan extracted from the Laminaria abyssalis marine algae on the human T cell lymphotropic virus type 1 (HTLV-1)-induced syncytium formation. The experiments were carried out in HeLa cells cocultured with a HTLV-1-infected T cell line (C91/PL cells) in the presence of the sulphated polysaccharide at concentration below that corresponding to the ED50. The sulphated fucan inhibited almost 100% of the syncytium formation at concentration of 100 microg/mI and was still active (>95%) at a concentration of 25 microg/ml. It was also observed that the best inhibition occurred when the compound was added in the first 2 h of the cell-to-cell contact. This is the first report showing that a purified sulphated polysaccharide, extracted from marine algae, is able to inhibit the cell-to-cell contact essential for the spreading of the HTLV-1.