The dose response for elicitation of the hypersensitive reaction in potato tuber discs by arachidonic acid (AA) suggested saturation at higher concentrations. Glucans from Phytophthora infestans, inactive themselves as elicitors of the hypersensitive reaction, enhanced sesquiterpene accumulation and hypersensitive browning elicited by AA. Significant activity (seven times control values) was observed with 33 pmol AA/3.0-cm potato disc in the presence of glucans. Glucans did not affect accumulation of steroid glycoalkaloids, influence the timing or relative amounts of sesquiterpenes which accumulate, or affect recovery of AA added to potato discs. Glucans enhanced activity whether added to potato discs 18 h prior to AA, at the same time as AA, or 18 h after AA. Elicitor activity in the presence of glucans was evident with 20-carbon unsaturated fatty acids that had little or no elicitor activity in the absence of glucans. The position of double bonds had considerable influence on the specific activity of unsaturated fatty acids. The most active had a minimum of three double bonds in a methylene-interrupted series beginning with delta 5, e.g., delta 5,8,11. A delta 5 double bond conferred significant activity even if it was not part of a methylene-interrupted series. The 20-carbon chain length appeared optimal for elicitor activity. The 22-carbon chain acids had low activity, and 16- and 18-carbon acids were inactive. A free carboxyl group or easily transesterified group appeared necessary for activity. Arachidonyl alcohol had very low activity and arachidonyl cyanide was inactive. AA-containing phosphatidylcholine, lysophosphatidylcholine and monoacylglycerol were at least as active as free AA, AA-containing diacylglycerols were slightly less active than free AA, and triarachidonyl glycerol was inactive.