AbstractThis study examined 328 CFS sera in a study with 17 CCFP, 8 Gulf War Veterans (GWV), 24 Prostate Cancer (PC), and 52 normal sera in the modified Membrane Immunobead Assay (MIA) procedure for CTX. Three hundred and twenty‐eight CFS patients' sera were examined by the modified MIA with purified MAb‐CTX and 91.2% gave a titre ≥1:40. 76% of the 17 CCFP sera samples and 100% of the 8 GWV sera samples also had a titre ≥1:40. 92.3% of 52 normal sera showed titres of 1:20 or less, while 4 gave titres of ≥1:40. In addition, 41 sera were examined for Anti‐Cardiolipin (aCL) by a commercial ELISA procedure with 87.8% demonstrating IgM, IgM+IgA, or IgM+IgG aCL antibodies. These results showed mostly the IgM aCL antibody alone in the sera samples. In addition, 41 serum samples were examined for aCL, with 37 showing positive for aCL, representing 90.2% positive for the three disease categories examined: CFS, CCFP and GWV. Examination for antiMitochondrial‐M2 autoantibody (aM‐M2) in 28 patients (CFS (18), CCFP (5), and GWV (5)) was negative for aM‐M2. Inhibition analysis with antigens, CTX, CFS “Acute Phase Lipids”, commercial Cardiolipin (CL) and 1,2‐Dipalmitoyl‐sn‐Glycero‐3‐[Phospho‐L‐Serine] (PS) and antibodies, MAb‐CTX and aCL from patients' serum show that the phospholipids in CL and CTX are antigenically indistinguishable with antibodies MAb‐CTX and CFS‐aCL. Preliminary chemical analyses have shown the lipids to be phospholipids associated with CL of the mitochondria. We designate this “Acute Phase Lipid” comparable to “Acute Phase Proteins” (C‐reactive protein (CRP) and Serum Amyloid A (SAA)) in inflammatory conditions. J. Clin. Lab. Anal. 22:99–105, 2008. © 2008 Wiley‐Liss, Inc.