The bicyclic endoperoxide prostaglandin (PG) H2 undergoes nonenzymatic rearrangement not only to PGE2 and PGD2, but also to levuglandins (LG) E2 and D2, which are highly reactive gamma-ketoaldehydes. Isoprostanes (IsoPs) are PG-like compounds that are produced by nonenzymatic peroxidation of arachidonic acid. PGH2-like endoperoxides are intermediates in this pathway. Therefore, we explored whether the IsoP endoperoxides also undergo rearrangement to form IsoLGs. Oxidation of arachidonic acid in vitro resulted in the formation of abundant quantities of compounds that were established to be IsoLGs by using mass spectrometric analyses. However, the formation of IsoLGs could not be detected in biological systems subjected to an oxidant stress. We hypothesized that this was due to extremely rapid adduction of IsoLGs to proteins. This notion was supported by the finding that LGE2 adducted to albumin at a rate that exceeded that of 4-hydroxynonenal by several orders of magnitude: >50% of LGE2 had adducted within 20 s. We therefore undertook to characterize the nature of LG adducts. Using liquid chromatography electrospray tandem mass spectrometry, we established that LGs form oxidized pyrrole adducts (lactams and hydroxylactams) with the epsilon-amino group of lysine. Oxidation of low density lipoprotein resulted in readily detectable IsoLG adducts on apolipoprotein B after enzymatic digestion of the protein to individual amino acids. These studies identify a novel class of ketoaldehydes produced by the IsoP pathway that form covalent protein adducts at a rate that greatly exceeds that of other known aldehyde products of lipid peroxidation. Elucidation of the nature of the adducts formed by IsoLGs provides the basis to explore the formation of IsoLGs in vivo and investigate the potential biological ramifications of their formation in settings of oxidant injury.