BACKGROUND The blood-cerebrospinal fluid barrier (BCB) has selectivity for protein components with different molecular weights. Protein components in the cerebrospinal fluid (CSF) change when the BCB is damaged. We calculated the alpha2 macroglobulin (alpha2M) index as an indicator of BCB permeability from the point of view of molecular weight and evaluated the relationship between the alpha2M index and CSF concentrations of the inflammatory biomarkers interleukin-6 (IL-6), C-reactive protein (CRP), and serum amyloid A (SAA) in Japanese subjects with infectious meningitis, in order to determine the clinical significance of those inflammatory biomarkers in CSF. METHODS IL-6, CRP, and SAA levels in CSF and serum were measured using various methods. The alpha2M index was calculated as the ratio of alpha2M (CSF/serum) to albumin (CSF/serum). RESULTS CSF IL-6 levels were higher than serum IL-6 levels in 16 patients with infectious meningitis. The difference in CSF IL-6 and CRP levels between mycotic or bacterial meningitis cases and healthy controls and in CSF SAA levels between all infectious meningitis cases and healthy controls were significant. There was a significant positive correlation between CSF levels of CRP or SAA and alpha2M indices. CONCLUSIONS Markedly increased levels of IL-6 in the CSF of patients with infectious meningitis may reflect the degree of intrathecal inflammation. On the other hand, increased CSF levels of CRP in patients with infectious meningitis, particularly mycotic or bacterial meningitis, and SAA in patients with all infectious meningitis may reflect the degree of damage to the BCB.