Interferon (IFN)-gamma was analysed immunologically in cerebrospinal fluid (CSF) sampled in the acute phase from 27 patients (15-66 years) with viral meningitis and from 18 patients (0.5-90 years) with bacterial meningitis. Increased CSF concentrations were observed in 19/27 viral and in 13/18 bacterial cases. CSF-IFN-gamma did not distinguish between viral and bacterial meningitis. Five of 8 patients with meningitis due to herpes simplex virus type 2 (HSV-2) had CSF-IFN-gamma levels above the highest found in enteroviral meningitis. Thus, a markedly increased CSF-IFN-gamma value in patients with suspected viral meningitis ought to indicate HSV-2 etiology. The patients with Streptococcus pneumoniae meningitis (6 adults and 1 child) had significantly higher levels than the 7 children with Haemophilus influenzae meningitis. This may indicate that S. pneumoniae induces more IFN-gamma secretion than H. influenzae, and/or that during meningitis, adults are more apt to react with IFN-gamma production, than are children.