Allergy to one of the most common Canadian storage mites, L. destructor, was demonstrated in a grain worker by immediate skin testing and inhalation challenge. The frequency of sensitization to this mite was 12% in 100 asthmatics by skin testing. In 4% of these asthmatics sensitization to another storage mite, Acarus siro, was found, all of whom were also sensitized to L. destructor and the house dust mite, Dermatophagoides farinae. There was a significantly higher proportion of asthmatics sensitive to the storage mites and to fungi who had a history of exposure to grain compared to those who did not.