Background: IgE-mediated hypersensitivity to fish is a clinically relevant problem, particularly in several European countries. Although most allergic reactions to fish are caused by ingestion, occupational exposures to seafood allergens by inhalation have been correlated with respiratory symptoms. In Madrid, patients with fish allergy have exhibited respiratory symptoms after visits to an open-air fish market. Objective: We sought to study the possibility of passively aerosolized fish allergen in an open-air fish market through air sampling and a competitive IgE immunoassay. Methods: Air samples were collected on polytetrafluoroethylene filters by using air samplers. Samples were collected on 41 different days from both an open-air fish market and an outdoor residential area. Fish allergens were specifically quantified by competitive IgE immunoassay by using pooled sera from fish-sensitive individuals. A raw fish extract (10 mg of dry weight/mL) was used as the reference standard. Results: Allergen was quantified in all 39 fish market air samples (2-25 ng/m³). The residential air samples contained no detectable allergen. The analytic limit of detection was 2 ng, allowing detection of 0.4 ng/m³ for the air volumes collected. A concentrated (30-fold) pool of fish market air samples was tested in serial dilutions and demonstrated an identical regression line to that of the raw fish standard. Conclusion: By using air sampling and an immunochemical analytic technique, fish allergen is detectable in the air of an open-air fish market. Avoidance of a food allergen, such as fish, should include preventing exposure to aerosolized particles through inhalation in relevant environments. (J Allergy Clin Immunol 2000;105:166-9.)