The Technical Committee of Classification and Labelling dealing with harmonized classification of substances and classification criteria under Directive 67/548/EEC on behalf of the European Commission nominated an expert group on skin sensitization in order to investigate further the possibility for potency consideration of skin sensitizers for future development of the classification criteria. All substances and preparations should be classified on the basis of their intrinsic properties and should be labelled accordingly with the rules set up in the Directive 67/548/EEC. The classification should be the same under their full life cycle and in the case that there is no harmonized classification the substance or preparation should be self-classified by the manufacturer in accordance with the same criteria. The Directive does not apply to certain preparations in the finished state, such as medical products, cosmetics, food and feeding stuffs, which are subject to specific community legislation. The main questions that are answered in this report are whether it would be possible to give detailed guidance on how to grade allergen potency based on the existing methods, whether such grading could be translated into practical thresholds and whether these could be set for both induction and elicitation. Examples are given for substances falling into various potency groups for skin sensitization relating to results from the local lymph node assay, the guinea pig maximization test, the Buehler method and human experience. © Blackwell Munksgaard, 2005.