Grapevine is susceptible to many diseases such as downy mildew, caused by the oomycete Plasmopara viticola (Pv). Sulfated laminarin PS3 is obtained by chemical sulfation of laminarin, an oligosaccharide (OS) from the brown algae Laminaria digitata. PS3 has been shown to elicit plant defense reactions and to induce resistance against Pv in grapevine (1). Our aim is to understand the penetration of OS in the grapevine leaf and to correlate it with the resistance induced against Pv. For this purpose, we followed the uptake of labeled compounds in the leaf, the effect of different adjuvants on their penetration and PS3 efficacy in Pv resistance tests. 1. S. Trouvelot, A. Varnier, M. Allegre, L. Mercier, F. Baillieul, C. Arnould, V. Gianinazzi-Pearson, O. Klarzynski, J. M. Joubert, A. Pugin, X. Daire Mol Plant Microbe Interact. 21, 232-243 (2008).