The detection of CSF oligoclonal IgG is one of the most useful laboratory tests to aid in the diagnosis of multiple sclerosis. Oligoclonal IgG is synthesized within the CNS and is usually detected in the CSF by agar and agarose gel electrophoresis or by isoelectric focusing. Oligoclonal IgG is present in about 90-95% in patients with clinically definite MS. There is a predominance of kappa light chains and IgG1 and IgG3 subclasses in oligoclonal IgG in MS. The specificity of oligoclonal IgG in this disease is still unknown, as antibodies, for instance against measles virus or myelin basic protein, represent only a minimal part of intrathecally synthesized IgG.