Allergy to the house dust mite Dermatophagoides pteronyssinus is mediated by IgE to the major allergens Der p I and Der p II in the majority of mite-allergic patients. In recent years, standardized preparations of D. pteronyssinus, commercially available from several sources, have become widely used for the diagnosis and immunotherapy of mite allergy. As standardization implies uniformity of allergen composition and potency, we directly compared the absolute and relative quantities of Der p I and Der p II in six different commercial standardized extracts of D. pteronyssinus. Our findings reveal variability in levels of both Der p I and Der p II, producing ratios of Der p I/Der p II ranging from 1.1/1 to 6/1. Although the content of minor allergens in the extracts was not evaluated here, their contribution to the overall reactivity of mite-allergic patients to the commercial extracts was judged to be minimal. This was demonstrated by showing that plasma depleted of reactivity to both Der p I and Der p II had virtually no residual IgE directed against extract components. The variation in the proportion of Der p I and Der p II among different D. pteronyssinus extracts is likely to influence their biological effectiveness. Patients with reactivity against only Der p I or Der p II, who were found to comprise approximately one-third of the mite-allergic population, may not respond optimally to extracts containing relatively low levels of the allergen to which they are sensitive.