Immunoglobulin G (IgG) was isolated from three multiple sclerosis (MS) and two control brains by Protein A Sepharose affinity chromatography and was characterized by thin-layer polyacrylamide gel isoelectric focusing (PAG IEF) and antiserum immunofixation. The three MS brains contained oligoclonal IgG. Immunofixation with measles, herpes simplex, varicella, rubella, mumps and cytomegalovirus as antigens and autoradiography revealed that some of the oligoclonal IgG bands separated by PAG IEF contained antibodies against herpes simplex in one, measles in two and varicella virus in all three MS brains. No antibodies were detected with this technique against structural human (crude saline, lipid-proteolipid, ganglioside, and myelin basic protein extracts of MS and normal human brain) and bovine (purified myelin myelin basic protein and oligodendrocytes of bovine brain) brain components. The finding of viral antibodies and the absence of antibodies against structural brain proteins in oligoclonal MS brain IgG is similar to that previously recorded in MS cerebrospinal fluid (CSF).