A management programme is described for a small colony of Obese strain (OS) chickens afflicted with spontaneous hereditary thyroiditis. Animals of this White Leghorn line are used as an animal model for Hashimoto's thyroiditis of man to study possible mechanisms of autoimmunity in general and organ-specific autoimmune diseases in particular. Due to the severe mononuclear cell infiltration of the thyroid glands, OS chickens show symptoms of hypothyroidism, including small body size, subcutaneous and abdominal fat deposits, long silky feathers, small combs and wattles, cold sensitivity, low fertility and poor hatchability. Successful breeding of this line, especially in a small population, can therefore be done only if rigid precautions are taken in aspects of animal care. The selection of breeding stock, the principal requirements for adequate housing and food, the artificial insemination procedure, and recommendations for collecting and incubating chicken eggs are reported in detail. Precautions necessary during the incubation of fertilized eggs, and fertility and hatchability are reported. During the hatching period several specific features must be considered. The important role of staff involved in a small chicken breeding unit is emphasized.