Using a monoclonal antibody in combination with the 4-step modified peroxidase-antiperoxidase (PAP) technique, Ia expression was demonstrated on endothelial cells and astrocytes in MS lesions of different ages. On endothelial cells, Ia antigen was found most frequently in grey and white matter parenchyma of acute MS brain and showed lower, comparable numbers in active and silent chronic MS brain tissue. Ia+ astrocytes were most numerous in acute MS lesions. In active chronic MS, Ia+ astrocytes predominated at the lesion edge, where they frequently displayed an atypical, rounded configuration. Positive astrocytes showed somewhat lower numbers within the lesion center and in normal white matter close to the lesion edge. Their frequency was significantly lower in normal white matter remote from the lesion. In silent chronic MS, Ia+ astrocyte processes were detectable only within the lesion center. Grey matter astrocytes displayed no staining with anti-Ia antibody. In normal brain tissue, no Ia antigen could be detected. The presence of Ia molecules on some endothelial cells and astrocytes in MS brain tissue suggests a role in antigen presentation perhaps relevant to the initiation and perpetuation, respectively, of the inflammatory process.