Fucans, sulfated polysaccharides extracted from brown seaweeds, have been shown to be endowed with inhibitory effects cell growth in various experimental models. We studied both the antiproliferative and antitumor properties of a fucoidan extract (HF) obtained from the brown seaweed Ascophyllum nodosum on a cell line derived from a non-small-cell human bronchopulmonary carcinoma (NSCLC-N6), this type of carcinoma is particularly chemo-resistant. HF exerts in vitro a reversible antiproliferative activity with a block observed in the G1 phase the cell cycle. Studies performed with the NSCLC-bearing nude mice show antitumor activity at subtoxic doses. These preliminary results indicate that HF exhibits inhibitory effect both in vitro and in vivo and is very potent antitumor agent in cancer therapy.