Patch testing with 7 irritants has been performed on a panel of 42 health volunteers, with the aim of determining concentrations which would induce mild to moderate reactions in at least 75% of individuals tested. The irritants studied and their optimum concentrations were as follows: benzalkonium chloride, 0.5%; sodium lauryl sulphate, 5%; croton oil, 0.8%; dithranol, 0.02%; nonanoic acid, 80%; propylene glycol, 100%; sodium hydroxide, 2%. Responder rates lower than 75% had to be accepted for benzalkonium chloride and sodium hydroxide in order to prevent overly severe reactions, whilst propylene glycol proved to have only marginal irritant properties.