Synthetic monosodium urate monohydrate crystals identical by x-ray diffraction powder pattern to crystals purified from a gouty tophus were sterilized and injected into the knees of two normal (non-gouty) subjects and into the stifle joints of 20 healthy dogs. Swelling, tenderness, warmth and redness of overlying skin and finally pain ensued (three-legged gait in dogs). Crystals were identified in synovial fluid neutrophils. Inflammatory intensity and neutrophilic response were directly proportional to the dose of injected crystals. Characterization of the suspected aetiologic agent, reproduction of the signs and symptoms in subjects not affected with the disease, and recovery of the aetiologic agent from the lesions fulfilled Koch’s postulates for proof of aetiology, albeit in a metabolic, rather than an infectious disease.