Updated project metadata. Methylglyoxal (MG) is a toxic byproduct of the glycolytic pathway and is quantitatively the most important precursor to advanced glycation end-products (AGEs). Insight into which proteins and in particular their individual modification sites are central to understand the involvement of MG and AGE in diabetes and aging related diseases. Here, we present a method to simultaneously monitor protein AGE formation in biological samples by employing an alkyne-labeled methylglyoxal probe. We apply the method to blood and plasma to demonstrate the impact of blood cell glyoxalase activity on plasma protein AGE formation. We move on to isolate proteins modified by the MG probe and accordingly can present the first general inventory of more than 100 proteins and 300 binding sites of the methylglyoxal probe on plasma as well as erythrocytic proteins.