The role of storage mites Acarus siro, Lepidoglyphus destructor, and Tyrophagus putrescentiae, compared with that of cow dander, as a cause of allergic rhinitis was studied by nasal challenges in a population of 106 non-smoking dairy farmers. Thirty-six of the subjects were challenged with storage mites alone, each with only one of the three species in consecutive order, 43 with cow dander alone, and 27 with both. Thus mite challenges were performed on 63 farmers and cow dander challenges on 70. Challenges with any one of the storage mite species were positive in 18% and with cow dander in 20% of farmers with allergic rhinitis. All challenges to asymptomatic farmers were negative. Five of the eight positive reactions to storage mites were due to Lepidoglyphus destructor, and three to Tyrophagus putrescentiae. Acarus siro was not responsible for any of the positive reactions. The results indicate that, among dairy farmers, storage mites are as common as cow dander as a cause of allergic occupational rhinitis.