Beta2 glycoprotein I (β2GPI) is an abundant plasma protein composed of 5 domains. The C-terminal disulphide bond is susceptible to cleavage by oxidoreductases. Certain biological functions of β2GPI depend on the redox form of the molecule. Nitric oxide metabolism has been shown to be disrupted in APS patients. We found that nitration of β2GPI occurs in-vivo and in-vitro in patients with APS and SLE and the nitrated form of the molecule alters its biological function. A novel assay was developed to quantify nitrated β2GPI levels. These were raised in patients with autoimmune disease compared to healthy and clinical event controls.