Abstract A heparin precipitable fraction (HPF) was obtained upon cooling (4°C) of heparinized plasma from certain dermatological patients, mainly affected with psoriatic arthritis. HPF was isolated by centrifugation at 4°C, washed at 4°C with 0,15 M NaCl and redissolved in 0,15 M NaCl at 37°C. In the non-clottable part of HPF, after clotting with thrombin at 37°C, one major protein band was reproducibly observed on SDS gel electrophoresis, the socalled X-component. In addition, small amounts of albumin and factor VIII related antigen (ristocetin cofactor) were also present. These proteins could be separated from the X-component by gel filtration on Sephadex G 150 or BioGel A-15m. The X-component is a glycoprotein, consists of at least two polypeptide chains each of around 220,000 in molecular weight, and shows immunological identity to cold insoluble globulin (CIG). This protein was incorporated into the fibrin clot when HPF was clotted at 4°C but not at 37°C. A simple procedure for the preparation of an antiserum against the X-component/CIG is described, and the interaction of this protein with fibrinogen and fibrinogen-derived complexes, especially in regard to the HPF phenomenon observed under pathological conditions, is discussed.