•  Effects of two algal polysaccharides, laminarin and carrageenans, on defence responses and signalling in tobacco plants is presented. A possible role as defence elicitors is important in the context of the use of algal extracts as plant protectants. •  The effect of the extracts was assessed after infiltration of tobacco leaves, and compared to the effect of a known elicitor of Phytophthora parasitica var. nicotianae(Ppn). •  Of the two algal polysaccharides, only carrageenans efficiently induced signalling and defence gene expression in tobacco leaves, as observed with Ppn elicitor. λ-carrageenan, with its high sulphate content, proved the most active. Defence genes encoding sesquiterpene cylase, chitinase and proteinase inhibitor were induced locally, and the signalling pathways mediated by ethylene, jasmonic acid and salicylic acid, were triggered. Some effects lasted for at least a week. •  λ-Carrageenan can elicit an array of plant defence responses, possibly through an effect of its high sulphate content. This helps clarify the mechanism of plant protection by algal extracts.