Techniques developed for protein blotting have been applied to the characterisation of allergen extracts. Such extracts, used for the diagnosis and therapy of allergic diseases, are highly variable in their content of active components. Protein blotting allows the direct identification of those components in an extract which bind human immunoglobulin E and cause allergic symptoms. It also allows the components to be defined in terms of molecular weight, isoelectric point and the frequency and intensity of IgE-binding by individual allergens. Because of the large number of samples which can be handled in a single experiment, protein blotting of allergens allows the identification of the individual allergen recognition spectra of large numbers of allergic patients. The resolution of components, loss of antigenicity, and aspects of blocking and probing which are important in identifying IgE-binding components are reviewed. A table of allergen sources investigated by protein blotting is included and important sources of allergens, such as pollens, mites and fungi are discussed in detail.