It has been proposed that susceptibility of potato to Phytophthora infestans would be a consequence of suppression and /or delaying of defense reactions by a soluble glucan which is released by compatible races of the fungus. In this report, the reaction of potato tuber slices (Solanum tuberosum cv. Huinkul) infected with either race I (1,4,7,8,10,11) or C (1,4,10,11) of Phytophthora infestans was studied. Race C grew better on slices than race I. Glucans from both races were isolated and their effect on the accumulation of phytoalexins and glucanases in tuber slices was studied. The glucans from the less virulent race (I) did not affect the accumulation of phytoalexins and glucanases in tuber slices infected or elicited with eicosapentaenoic acid, whereas the glucans from race C produced 70% inhibition of phytoalexin accumulation and reduced by 50% the induction of glucanase activities. Purified glucanases from potato degraded the glucans from race C but not from race I. The results reported here show that, at least on this cultivar, glucans from both races affected defense responses in a different manner, which could reflect structural differences between these glucans.