Morphological and autoradiographical studies of leukocyte cultures of the CSF in 20 patients with serous meningoencephalitis of a diverse etiology permitted the authors to establish the following condition. There is a phenomenon of lymphocyte blastransformation under the influence of phytohemagglutinine (PHA) and tuberculin. These data made it possible to suppose of the existence of T2-lymphocytes in the CSF. A correlation between the obtained data and the clinical picture demonstrated that 30-50% of the lymphocytes respond to PHA, irrespective of the etiology of meningoencephalitis, while blasttransformation to tuberculin occurs only in tuberculous meningoencephalitis. The authors attribute an important practical role to these findings for a differential diagnosis between obliviated forms of tuberculous meningoencephalitis and serous meningoencephalitis of a nontuberculous origin. These data give ground to assume that the CSF and the lymphocytes circulating in it form a strong immunological "mobile" barrier to the brain.