A rapid and simple procedure for the preparation, directly from clotted whole blood, of bovine fibrin free of cell membrane contaminants is described. The procedure exploits the ability of detergents to solubilize and extract cell membrane proteins from homogenized clot fragments. The non-denaturing detergents, TX-100* and DOC, used in sequence, or the denaturing detergent, SDS, used alone, proved equally effective. The resulting fibrin • has the solubility properties and the subunit composition on SDS gels of highly cross-linked fibrin. © 1980, Taylor & Francis Group, LLC. All rights reserved.