The peroxidase-immunoperoxidase immunocytochemical method was used on 27 saphenous vein coronary artery bypass grafts, which had been resected because of recurrent angina, to identify in situ cellular and humoral elements possibly associated with graft occlusion. Immunostaining was performed on paraffin wax embedded control saphenous vein and graft sections incubated directly with primary antibodies against von Willebrand antigen (vWFAg), fibronectin, fibrinogen, leucocyte common antigen (LCA), lysozyme, vimentin, desmin, platelet factor 4, and thrombospondin. Antigens were visualised by a chromogen providing an orange-red immunoprecipitate at the site of epitope localisation. The intraluminal, amorphous exudate present in most grafts was not composed simply of fibrin or fibrinogen, as previously thought, but was a multiprotein complex including wWFAg, fibronectin, thrombospondin and platelet factor 4. Along with macrophages, these components probably enter the graft after haemodynamic, physical, and chemical injury to, and disruption of, the endothelial cell. Progressive myointimal proliferation and fibrosis of these grafts may be local repetitive responses to macrophages and platelets, cells previously known to participate in vascular disease.