Model systems used in the determination of serum antioxidative activity (AOA), which differ both in the way of generating free radicals and in the mode of their detection, are clinically analyzed. The specific features and potentialities of the model systems developed at the authors' laboratory are characterized. These included yolk lipoprotein suspensions, liposomal suspensions formed from total phospholipid fraction, the hemoglobin-hydrogen peroxide-luminol system. The investigations show that most model systems for determining serum AOA contribute to the water soluble interceptors of free radicals (ascorbate, urate, plasma proteins, etc.), chelating and oxidative agents of catalytically active Fe2+ (ceruloplasmin, transferrin, albumin, etc.). The serum AOA levels measured with different model systems vary with the body's status. To determine serum AOA and the contribution of major endogenous antioxidants and inhibitors of free radical reactions may be a basis for the goal-oriented use of exogenous antioxidants in the therapy of a great variety of diseases.