Infection of bovine cells with bovine viral diarrhoea virus (BVDV) can be blocked by the monoclonal antibody (mab) BVD/CA 26, which is directed against a cellular membrane protein. To characterize this molecule, it was isolated and purified by column chromatography. It was found to be an acidic, glycosylated membrane protein consisting of two polypeptide chains of about 28 and 56 kDa. Under non-reducing conditions the chains formed multimers of about 200 kDa. In an actin binding assay the 56 kDa polypeptide chain bound to F-actin as judged by co-sedimentation with actin filaments. Since the target molecule of BVD/CA 26 is localized on the surface of living cells and additionally binds to F-actin, a possible biological function may be to connect the cortical actin filaments with the cellular plasma membrane. The blocking effect of BVD/CA 26 indicates that this cellular plasma membrane protein is involved in the endocytic pathway of BVDV particles.