1. The inhibitory (antioxidant) effect of serum on the autoxidation of tissue polyunsaturated fatty acids has been investigated.
2. Dialysis studies, DEAE-cellulose chromatography, gel filtration, immunoelectrophoresis and immunodiffusion experiments showed that this effect is the function of two serum protein fractions.
3. One fraction was identified as transferrin. The second fraction includes caeruloplasmin.
4. Studies with radioactively labelled α-tocopherol showed that serum tocopherol in the physiological concentration range contributes only marginally to the total antioxidant activity of serum.
5. These findings are discussed in the wider context of biological antioxidant protection.