Immunoglobulin G (IgG) in soluble and particulate fractions of discrete tissue samples from multiple sclerosis (MS) brain was analysed. Supernatant IgG/albumin rations and particulate-bound IgG levels were highest in samples dissected from MS plaques and adjacent white matter. Acid extracts of particulate fractions from the equivalent of 1 g of MS plaque tissue contained up to 15 micrograms IgG, an order of magnitude less than the amount in extracts from a subacute sclerosing panencephatitis (SSPE) brain, but 20 times more than those from control brain. By contrast, supernatant fractions from SSPE brain and some MS plaques contained comparable amounts (100-200 micrograms/g tissue) of IgG, which were 10 times greater than those from control brain. Samples were subjected to isoelectric focusing (IEF), and IgG was visualized by immunoperoxidase staining. The IEF patterns of IgG from control samples were diffuse but samples from demyelinated MS tissue displayed distinct oligoclonal bands of IgG. A number of common IgG bands were apparent in extracts of supernatant and particulate fractions from the same plaque. The IEF spectra of plaque samples from three MS brains were different. Furthermore, quantitative variations in certain IgG bands were observed in different plaques from the same brain.