Measurements were made of the granulocyte elastase activity and the concentration of granulocyte elastase alpha 1-proteinase inhibitor complex (E-alpha 1 PI) in the cerebrospinal fluid (CSF) of patients with purulent meningitis and aseptic meningitis. Granulocyte elastase activity was determined by the cleavage of methoxysuccinyl-L-(alanyl)2-prolyl-valyl-p-nitroanilide peptide substrate. The E-alpha 1 PI concentration was assayed with an enzyme immunoassay technique. Granulocyte elastase activity in the CSF on admission was significantly higher in patients with purulent meningitis (800 +/- 539 micrograms/L; mean +/- SD) than in patients with aseptic meningitis (< 10 micrograms/L). Granulocyte elastase activity in the CSF of patients with purulent meningitis decreased rapidly except for two patients who subsequently developed hydrocephalus. There was a weak correlation between granulocyte elastase activity and the concentration of E-alpha 1 PI in the CSF with purulent meningitis (n = 67, r = 0.48, P < 0.01). However, some samples of CSF showed a marked discrepancy between granulocyte elastase activity and E-alpha 1 PI concentration, indicating the importance of measuring granulocyte elastase activity in the CSF to estimate tissue injury.