Nasal secretions from eighteen patients with allergic rhinitis with a positive case history, intradermal test and nasal provocation test, but with negative or only weakly positive RAST (radioallergosorbent test) on the serum against a total of thirty-five allergens, were studied. In the RAST an immunosorbent-purified anti-IgE with DE2 specificity was used, which raised the detection limit. Nasal secretion was collected by washing the nasal mucosa with 0-9% and 18% NaCl solution respectively, and the latter secretion was also luophilized and concentrated. In ten cases RAST was slightly positive on the nasal secretion, and in three of the concentrated secretions the RAST value was higher than on the serum. In none of the serum or nasal secretion samples was RAST positive according to the cut off value for a positive result defined by the reference system used in Phadebas Rast. From these results it is concluded that RAST analyses of nasal secretion from patients with allergic rhinitis is of no appreciable value in routine clinical allergological diagnosis. However, the increased sensitivity of RAST obtained with isotope-labelled anti-DE2 may be useful in the serological diagnosis of patients with low grade allergy having low levels of IgE antibodies in serum.