Quantitative determination of alpha interferon (IFN) is used as an early marker in viral encephalitis. IFN is detected during 10 days following the onset of clinical symptoms. In 26 patients (11 children from 1 day to 6 year old and 15 adults from 17 to 70 year old) with central nervous system disorders (15 meningo-encephalitis, 5 meningitis, 1 myelitis, 1 polyradiculoneuritis, 1 dementia, 1 epilepsy and 2 other), alpha IFN is quantified using a cytopathic effect inhibition assay of VSV on MDBK cells. The mean value of alpha IFN is 80 UI/ml (range from 0 to 512 UI/ml). Virus involved are herpes virus in 38.5% of cases (10/26) and 66% of viral meningoencephalitis (8/12), H.I.V. in 3 cases, VZV in 2 and measles virus in 1 case. Viral aetiology is suspected in six other patients. The results show the importance of early determination of alpha IFN (immediately after the first symptoms and on the first admission to the hospital) in sera and cerebrospinal fluids (CSF) simultaneously with viral culture and antibody research. The presence of alpha IFN only in CSF and a higher titre of alpha IFN in CSF than in serum are important data to distinguish primitive acute necrotizing encephalitis from post eruptive or post infectious perivenous encephalitis. In herpes virus infections with specific treatment all the patients recover. However to prevent brain damage in survivors the treatment should be established as soon as possible.