Experimental and chemical investigations revealed that common ivy (Hedera helix susp. helix) contains 3 compounds which are powerful irritants and moderate sensitizers. Only 2 of these constituents, falcarinol and didehydrofalcarinol, are present in the plant during the whole year. Besides Panax ginseng and Schefflera arboricola, this is the third species of the Araliaceae in which these polyacetylenic sensitizers have been found. Falcarinol and didehydrofalcarinol also occur in Hedera helix subsp. canariensis. 4 patients have been patch tested. Even in low concentrations (0.03%), the main allergen falcarinol elicited strong reactions in all of them. One of the authors became sensitized during the investigations.