To investigate blood-barrier leakage of plasma proteins in acute plaques of multiple sclerosis (MS) the authors used immunocytological methods to examine frozen tissue removed at autopsy from recently active cases. Annular patterns of protein-rich leakage were seen which may help to elucidate the patterns observed using gadolinium-enhanced nuclear magnetic resonance imaging. Vessel wall damage was found in all acute plaques examined and this was associated with the intramural deposition of complement on smooth muscle components and with an infiltration of HLA-DR +ve macrophages. In addition, all acute cases examined had small plaques which contained particulate material within macrophages and astrocytes, on which complement and immunoglobulins colocated. Attempts to find similar material in cases of chronic MS, subacute sclerosing panencephalitis and in perivenous encephalomyelitis were unsuccessful. These results suggest that the inflammatory changes in early MS plaques may have some specificity which could be related to the antigens whose presence is inferred by the colocation of complement and immunoglobulin on material within activated macrophages and astrocytes.