Peroxidase-labelled Ulex europeus I lectin has been applied to a series of routinely fixed and processed vascular lesions to assess its value in the identification of endothelium. A group of breast carcinomas previously examined for binding of the lectin to the tumour have been re-assessed for evaluation of the use of Ulex europeus I in the determination of vascular invasion. There was consistent binding to normal vascular endothelium but this could be variable, there being stronger reactivity in small vessels of some tissues. All benign lesions stained showed clear definition of vascular channels but angiosarcomas were much less consistent in their staining reaction and the peroxidase-labelled lectin was considered to be a less satisfactory marker. Lymphatic endothelium was always negative; this proved to be a useful feature in differentiating between blood vessel and lymphatic invasion by breast carcinoma. Ulex europeus I-peroxidase could certainly have a role in routine histopathology in this area.