Summary: The toxicity of phenolic substances for the skin of normal guinea pigs required at least two structural features. The hydroxyl groups must be free and, if two are present, properly oriented; phenols and catechols could be toxic but resorcinols were not. In addition, an n-alkyl side chain with five or more carbon atoms was required with cutaneous toxicity increasing to a maximum at a chain length of 15 to 17 carbon atoms. Substitution of a 1′ OH into the side chain of 3-n-pentadecylcatechol eliminated the toxicity of this compound.