The cloning and production of an increasing number of allergens through the use of DNA technology has provided the opportunity to use these proteins instead of natural allergen extracts for the diagnosis and therapy of IgE-mediated allergic disease. For diagnostic purposes, it is essential that the molecules exhibit IgE-reactivity comparable with that of the natural wild-type molecules, whereas T cell reactivity and immunogenic activity may be more important for allergen-specific immunotherapy. In relation to the latter, the development of hypoallergenic recombinant allergen variants is an approach which shows great promise. Clinical application of the proteins requires that they must be produced under conditions of Good Manufacturing Practice and meet the specifications set down in the appropriate Regulatory Guidelines, principally the ICH-Guidelines. Special consideration has to be given to the choice of expression system, the design of the expression vectors, and the purification strategy to obtain a pure product free from toxins and contamination. The availability of the pure recombinant molecules provides the opportunity to formulate preparations that are free from the non-allergenic ballast proteins present in natural allergen extracts and which contain relative concentrations of the allergens in clinically appropriate proportions.