Allergy to animals affects between 15 to 30% of atopic persons and about the same proportion of all those who work with laboratory animals. The responsible allergens are contained in the urine, saliva, and secretions of furred animals. The allergens apparently dry on fur, bedding, or other fomites so that with disturbance, the antigen becomes airborne on particles or "vectors" of diverse size. Many of these vectors are small enough to remain airborne and reach the lower airway. They adhere to surfaces in the environment such as rugs, walls, and clothing, so that animal allergens are found in virtually every sample of household dust and in the air and surfaces throughout most research facilities. This creates a reservoir of allergen that makes total elimination of the allergen from any environment very difficult. It then becomes important to understand the minimum level of allergen capable of inducing symptoms in sensitized patients so strategies can be developed to reduce exposures to below these limits. Otherwise, management will depend on nearly complete avoidance of animal dander, a difficult task. Alternatively, it will be necessary to modify the allergic host with drugs or immunotherapy.