In this study, the antigenic expression of CD34, a 110 kDa glycoprotein which is expressed on human haemopoietic progenitor cells and vascular endothelium, has been assessed in a variety of neuropathological conditions, including infectious and demyelinating disease. Using immunoperoxidase staining on paraffin sections, the immunohistochemical results show that CD34 antigen is expressed widely on human CNS endothelium in grey and white matter, in the eye including retina, and in the anterior and posterior lobes of the pituitary. In demyelinating disease CD34 antigen expression was not detected in acute lesions, whereas strong expression was observed in old lesions. CD34 endothelial positivity was observed in areas of gliosis, vasogenic oedema, vascular disease and in Alzheimer's and Parkinson's disease pathology. A general pattern emerged, with CD34 antigen reactivity predominantly negative in areas of inflammation with demyelination but positive in adjacent non-inflamed tissue, irrespective of myelin pathology. We conclude that perivascular inflammation is a key factor in the absence of immunoreactivity of CD34 in the CNS in demyelinating disease.