Our investigations concerned the blood-brain barrier (b.b.b.) in patients with acute bacterial purulent meningitis. For that purpose concentrations of proteins, which are synthesized beyond the central nervous system and in normal cerebrospinal fluid (CSF) exist only in slight amounts, were determined in CSF and in blood serum. Albumin was examined in the CSF of 59 patients and in the serum of 35 of them, transferrin of 40 and 32 patients, respectively. Etiological verification was obtained in 84.7% of patients. The control group consisted of 20 persons. Quantitative analytical tests were carried out by means of immunochemical, turbidimetric methods. High levels of albumin and transferrin in CSF and low in serum of patients with meningitis were observed. The obtained results, confirmed by statistical analysis, demonstrate that in the acute phase of purulent meningitis b.b.b is impaired, what leads to the transfer of the proteins from the blood serum into the cerebrospinal fluid and that transferrins a better indicator of the damage to blood-brain barrier than albumin.