Twenty-six Dermatophagoides pteronyssinus (D.pt.) sensitive subjects were skin prick tested in duplicate with 15 concentrations of D.pt. ranging from 0.0018 to 17.8 mg/ml, 15 concentrations of the major allergen of D.pt., antigen P1, ranging from 0.0002 to 1.88 mg/ml and 15 concentrations of histamine dihydrochloride solution ranging from 0.048 to 114.0 mg/ml. Weal areas and concentrations were transformed by taking logs and linear and non-linear regression curves fitted, allowing for confounding variables, such as subject, and interactions. The weal areas over all concentrations fitted "S" shaped curves with essentially straight central portions, parallel between materials, with differences between subjects but parallel within subjects. The dose response curves of P1 and D.pt. were coincident when the concentrations were adjusted to allow for differences in potency. The concentrations of allergen and histamine commonly used for standardisation purposes will give weals that can be plotted along a straight line, but at higher and lower concentrations the response will tail off. This accounts for previously ambiguous results. Standardisation of allergens using 10 mg/ml histamine is preferable to 1 mg/mg.