A rabbit immune serum against SSPE CSF reveals in these CSF, in addition to IgG, another protein with electrophoretic gamma-mobility which is never revealed by anti-normal human serum nor by anti-normal CSF. This protein, called B.T., has been found in 5 out of 8 CSF studied. The same immune serum against SSPE CSF reveals in normal brain extracts a specific protein sharing antigenic identity with the fraction B.T. from SSPE CSF. This brain protein is located in the white matter. It has not been found in the grey matter, nor in other tissue extracts. Due to its solubility and electrophoretic properties, this brain protein B.T. is not comparable with any of the brain specific proteins known at this time.