As part of the studies to establish an international reference preparation of Dermatophagoides pteronyssinus allergens, ten coded extracts of this mite were assessed in four laboratories. The extracts were compared for total potency using direct RAST, RAST inhibition and quantitative skin tests, and also for composition and major allergen content using crossed immunoelectrophoresis, crossed radioimmunoelectrophoresis, rocket immunoelectrophoresis and radioimmunoassay. In addition, the source materials were examined by light microscopy, and the extracts were examined for the presence of proteins/allergens derived from the culture media. To help with standardization, a new reference pool of sera from patients allergic to D. pteronyssinus was also established (National Institute of Biological Standards and Control, NIBSC, 82/528). The results showed that techniques are available for measurements of potency and allergen content. In several of the extracts, culture medium derived allergens and antigens were demonstrated. It also became clear that extracts varied not only in their total potency but also in the distribution of the identifiable allergens. In particular, extracts derived from isolated mites contained more AgX and/or Ag 23 relative to their content of antigen P1 (= Ag42). These studies lead to the choice of an extract for an international reference preparation (NIBSC 82/518) and helped to establish the methods used subsequently in the international collaborative study.