Immunoglobulin G (IgG) was purified by single-step protein A-Sepharose (Pharmacia) affinity chromatography from the cerebrospinal fluid (CSF) of multiple sclerosis (MS) patients and controls. Autoantibodies to myelin basic protein (anti-MBP) were isolated from the purified IgG fraction by two-step antigen specific affinity chromatography. Anti-MBP in the context of whole CSF or in purified form reacts equally to MBP prepared from non-MS or MS brain tissue. Kinetic studies of anti-MBP titers demonstrate that when anti-MBP is reacted with increasing amounts of non-MS or MS MBP, the autoantibody is immunoabsorbed by either antigen in vitro. Immunoabsorption of anti-MBP by MBP or its synthetic peptides may also be possible in vivo as a potential therapeutic tool.