Positive results were attained when human peripheral blood lymphocytes (PBLs) were investigated for their ability to propagate a transmissible cytotoxic activity (TCA) isolated on VERO cell cultures from a sample of cerebrospinal fluid (CSF) drawn from a woman with ischemic brain injury. In consideration of this finding it can be assumed that "in vivo" blood lymphocytes contributed to give rise to the TCA detected "in vitro" in the CSF inoculum.