The allergen skin prick test has been optimised for allergen assay for both D. pteronyssinus and grass pollen extracts. The use of an iterative algorithm to select the linear portion of the transformed dose response curve gave an assay that was stable to changes both in the composition of the panels of atopic subjects and in the time intervals between challenge and response measurement. The coefficient of variation of the assay was 18%. The overall robustness of the assay was shown by an isothermal stability trial in which glycerinated D. pteronyssinus remained comparable with a freeze-dried reference preparation at 5 and 20 degrees C for 3 years. A comparison was also made between the first International Reference Preparation to D. pteronyssinus and 'in house' reference preparations. The skin prick test is thus a valuable bioassay for allergen measurement and can provide a reference assay for the calibration of alternative in vitro assays.