1. Fresh ox-brain homogenate prepared under standard conditions at 4°C and stored at −20°C autoxidizes spontaneously and reproducibly when re-heated to 37°C. 2. The preparation can be used for assaying the antioxidant activity of serum and of other biological fluids. The preparatory and assay procedures are described. 3. The antioxidant activity of normal serum, cerebrospinal fluid, synovial fluid and of various known antioxidants have been compared.