As a result of the development of a number of modern biochemical and immunological in-vitro techniques together with in-vivo skin tests, it has been possible to purify, characterise and standardise allergen extracts for diagnosis and therapy. Despite the diagnostic value of the NPT for daily clinical practice, no standardisation has been achieved to establish the correct allergen concentration for that test. In 43 patients allergic to grass we determined the nasal threshold by means of allergen titrations with increasing concentrations from 1000 BE/ml to 170,000 BE/ml (1000 BE/ml = 1 HEP). For evaluation we used rhinomanometry and symptom scores. 10 normal subjects served as control. In order to achieve the best sensitivity and specificity of the NPT, allergen concentrations of 100,000-150,000 BE/ml had to be used. For research purposes an allergen titration is recommended as standard. Much work remains to be done in respect of technique and evaluation criteria to establish a standardised and reliable NPT.