1. Plasma antioxidant activity was determined in rats, guinea pigs, ra
bbits and humans. 2. Low levels of antioxidant activity were observed
in rats and guinea pigs. Both species showed high susceptibility to li
pid peroxidation in red blood cells. 3. In rabbit and human plasma ant
ioxidant activity was high. In these species, susceptibility to lipid
peroxidation was low.