The occurrence of oligoclonal IgG and homogeneous free lambda light chains in the cerebrospinal fluid (CSF) of a patient with progressive rubella virus panencephalitis is reported. Fractions of oligoclonal IgG corresponding to those of the CSF occurred also in the serum.Evidence is presented that the oligoclonal IgG of serum and CSF represent rubella virus‐specific antibodies. Different populations of oligoclonal IgG appear to be associated with antibody specificities to different antigenic components of the viru