Extracts of five common atmospheric moulds (Alternaria, Aspergillus, Cladosporium, Mucor and Penicillium) obtained from different manufacturers were compared by means of skin prick testing and, partly, with the radioallergosorbent test. Considerable differences were found between the extracts with respect to positive tests found in mould-allergic individuals and controls. A purified allergen preparation of Cladosporium herbarum was superior to the crude allergen extracts. The results suggest that the allergen contents of the extract differed both with respect to total allergenic potency and to allergenic composition.