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Notes: Abstract A comparison of measles virus antibody titers in sera and CSF samples has demonstrated that antibodies appearing in the latter type of samples are locally produced in the central nervous system in all patients with SSPE and in about 60% of patients with MS. The relationship between these locally produced measles virus antibodies and the oligoclonal IgG in patients with these diseases was studied by use of two different techniques. One technique was to absorb concentrated CSF with purified measles virus particles to analyse their capacity to remove oligoclonal IgG. Antigen-antibody complexes formed were isolated by sedimentation whereafter IgG was eluted at pH levels of 4.0, 3.0 and 2.0. The second technique was to make a preparatory electrophoresis of concentrated CSF and then to analyse the distribution of antibodies against different measles virus antigens in fractions collected. In the case of SSPE repeated absorbtions with measles virus antigens caused an almost complete removal of all oligoclonal IgG. After preparatory electrophoresis peaks of antibody activities correlated in their electrophoretic position to the occurrence of bands of oligoclonal IgG. Different bands were interpreted to carry different antibody activities. Results of studies of materials from MS patients differed somewhat from those obtained in characterization of materials from SSPE patients. Only in exceptional cases could absorption with purified virus particles modify the band pattern of oligoclonal IgG and generally absorption with measles virus antigens in the selected cases did not influence the level of IgG in the samples. However in low pH eluates measles virus antibody activities and a few bands of oligoclonal IgG were recovered. Preparatory electrophoresis of concentrated CSF material from MS patients resulted in a varying distribution of antibodies against different measles virus antigens but a distinct correlation between accumulation of antibody activity and bands present in the electropherogram could not be discerned It is concluded that in the case of SSPE the oligoclonal IgG represents an oligoclonal IgG response to an intense hyper-immunization with measles virus antigens. In contrast a local immunization with measles virus antigens accounted only for a few bands of oligoclonal IgG in cases of MS whereas the potential antibody activity of the major part of oligoclonal IgG remains to be explained.