Blood-brain barrier (BBB) permeability for the blood lymphocytes was experimentally investigated along with the central nervous system and cerebrospinal fluid (CSF) autoimmunity. Labelled autologous lymphocytes were infused into the blood of 16 dogs and their CSF (cysterna magna) lymphocytes were radiometrically investigated in order to study the blood lymphocyte BBB permeability. Immunity and autoimmunity states were studied in 26 patients with acute cerebral infarction, 12 patients with serous meningitis and 10 with brain contusions. The data indicated that intact BBB is impermeable for the peripheral blood lymphocytes. Vascular, infectious and traumatic brain lesions deteriorate the BBB barrier that is conductive to peripheral blood lymphocytes penetration causing an autoimmune cellular and humoral response.