Milk has been long recognized as a particularly nourishing and adaptable food, but was limited in use until production and quality controls were instituted about 50 years ago. The good reputation of milk as a highly nutritious, high quality food is firmly established today but it must be continually guarded and strengthened. This article reviews the effects of mastitis on milk quality and discusses the value of quality control testing programs as they affect dairymen, manufacturers and consumers.