The antioxidants in the aqueous phase of human plasma include ceruloplasmin, albumin (the protein itself and possibly also albumin-bound bilirubin), ascorbic acid, transferrin, haptoglobin, and hemopexin. Assays that attempt to answer the question "what is the most important antioxidant?" are compared, it being concluded that the answer is different depending on the nature of the prooxidant stress imposed in the assay.