Fibronectin concentrations in the cerebrospinal fluid were assessed in 20 patients with acute meningitis using a turbidimetric immunoassay. A significant increase in fibronectin concentrations was observed in patients with bacterial meningitis; decreased concentrations were observed in patients with viral meningitis. The determination of fibronectin concentration in patients with bacterial meningitis may represent a useful marker in differentiating bacterial from viral meningitis.