This report describes the immunohistochemical detection of von Willebrand's factor (vWf) and CD31 antigen in paraffin wax-embedded, formalin-fixed tissue sections of canine normal organs and vascular neoplasms. CD31 antigen and vWf were detected in endothelial cells of all organs examined, except for the endothelia of the renal glomeruli, which were negative for vWf. All haemangiomas examined (15) were positive for both markers. Eleven of 15 haemangiosarcomas were positive for vWf and all 15 expressed the CD31 antigen. All other neoplasms investigated (fibrosarcomas, schwannomas, haemangiopericytomas) were negative for both markers. It is concluded that immunohistochemical detection of CD31 antigen is of value for studying vascular disorders of the dog in routinely processed tissue. © 1995 Academic Press Limited.