Oxidative stress has been clearly implicated in human disease by a growing body of scientific evidences. There is no ideal method for the measurement of this parameter. A possible strategy would be to measure simultaneously several biomarkers representing damage to different cellular components or, alternatively, a method able to evaluate the hydroperoxides, intermediate products of oxidation originating from different classes of molecules, such as lipids, peptides, amino acids, etc. can be used. We are introducing a simple, rapid and inexpensive assay to measure the oxidative status of human plasma. It is based on the properties of N,N-dimethyl-p-phenylenediamine (DMPD), a compound able to produce a fairly long-lived radical cation. The absorbance at 505 nm of a DMPD solution in the presence of plasma, which is proportional to the amount of hydroperoxyl compounds, is related to the oxidative status of the sample and could be expressed as hydrogen peroxide equivalents (HPE). This assay was not influenced by freezing-thawing and storage time of the plasma samples. The assay can be automated, performed in a kinetic mode, and used for routine analyses. The DMPD assay alone or in combination with analytical methods for assessing antioxidant capacity is suggested as a reliable tool to obtain information in pathologies related to oxidative stress.