The pro-inflammatory, leukocyte-derived S100A12 protein occurs as calcium-dependent oligomers in serum, while EDTA plasma from the majority of healthy individuals contains only monomers. Addition of 5 mM EDTA to serum leads to a rapid dissociation of the oligomers in most samples. However, using gel permeation chromatography, we have found that sera from some patients and seemingly healthy individuals contain molecular complexes in the 400-1000 kDa range reacting with anti-S100A12 even in the presence of EDTA; for these we introduce the name ERAC (EDTA Resistant S100A12 Complexes). Based upon monoclonal antibodies and the lateral flow principle, we have developed a quantitative rapid ERAC test giving results within 10 minutes. The highest prevalence of ERAC positivity was found in sera from patients with concomitant rheumatoid arthritis and coronary heart disease. The structure of ERAC is not yet known. Further studies are needed to analyse the mechanism behind the appearance of ERAC and the possible association with inflammatory-related diseases.