Nasal challenges with pollen grains represent one of the techniques of provocation. However, the clinical criteria of positivity are not clearly established. Nasal challenges with increasing numbers of orchard-grass pollen grains were performed in 60 patients allergic to grass pollens and 20 normal subjects. Before any challenge, the nose was washed three times with saline and then lactose, and 50, 150, 450, 1350, and 4050 orchard-grass pollen grains were insufflated into the nostrils until a symptom score of 5 was reached. This score was mainly based on major symptoms of allergic rhinitis, for example, rhinorrhea, nasal obstruction, sneezes, and to a lesser extent, on minor symptoms, such as pruritus, conjunctivitis, and pharyngitis. Nasal secretions were obtained after each challenge by lavage. Histamine was titrated by a radioimmunoassay with a monoclonal antibody against acylated histamine. Prostaglandin D2 (PGD2) was assayed with an enzyme immunoassay with a polyclonal antibody against PGD2 methoxamine. None of the normal subjects had a symptom score greater than 2; 55/60 patients had a positive challenge. The release of PGD2 was significantly (p less than 0.001, Kruskal-Wallis test) correlated with a symptom score of 5; 74.5% of patients had a significant release of PGD2 in nasal secretions. In contrast, although 58.2% of patients had a release of histamine in nasal secretions when the challenge was positive, the correlation with symptom scores was not significant. PGD2 in nasal secretions increased 3.7-fold after a positive nasal challenge.(ABSTRACT TRUNCATED AT 250 WORDS)