The proper functioning of aerobic organisms subjected to the danger of uncontrolled oxidation depends critically on the efficient antioxidant system. This system includes antioxidant enzymes, low-molecular weight antioxidants and biological chelators of transition metal ions. What is an antioxidant? According to the apparently most useful definition, antioxidant is “any substance that, when present at low concentrations compared with those of an oxidizable substrate, significantly delays or prevents oxidation of that substrate” [1]. The usefulness of this definition lies, i. a., in its instrumentality as it suggests straightforward means of testing antioxidant properties of chemical compounds. In fact, testing the delay or inhibition of a chosen oxidation reaction allows for a simple measurement of the sum of all antioxidants present in the sample studied. This parameter is usually referred to as Total Antioxidant Activity or, as recommended lately, Total Antioxidant Capacity [2]. Total Antioxidant Capacity is an increasingly frequently measured parameter. While lacking detailed information about the composition and concentrations of individual antioxidant species present in a sample, it provides an easy and rapid means of evaluation of “antioxidant power” of the examined material. An obvious field of application of TAC assay is analysis of plants, food products and beverages. However, there are many attempts to apply this analysis for evaluation of antioxidant status of body fluids (especially blood plasma) in normal human subjects and animals. For research purposes, it is also of interest to evaluate TAC of cells and tissue homogenates in animals subjected to different treatments. There are numerous methods of estimating TAC. One group of methods is based on the inhibition of free-radical induced oxidation of an indicator compound or of a substrate yielding an indicator [3–5]. The