Rapid diagnosis of viral infections in the central nervous system has become increasingly important. Antiviral treatment, prevention of spread of disease and differentiation from infections caused by agents sensitive to antibiotics may be the important consequences of a virus specific diagnosis gained early in the disease. The diagnosis can be obtained by detection of virus or viral antigen in the human specimen: herpes simplex virus by electron microscopy, immunofluorescence or immunosorbent assays in brain biopsies; rabies virus by immunofluorescence in corneal cells or skin and mucous membranes. The presence of measles or influenza antigens in nasopharyngeal secretions, shown by immunofluorescence or enzyme immunoassays, may diagnose an encephalitis caused by either of these viruses. Where suitable material is not available the detection of virus-specific IgM in a single serum specimen may be used for diagnosis. Mumps specific IgM activity is detected by enzyme-linked immunosorbent assay (ELISA) or indirect immunofluorescence techniques; tick-borne encephalitis (TBE) specific IgM by immunosorbent assays or by reduction of hemagglutination-inhibition (HI) titer by 2-mercaptoethanol treatment of serum. Reports have been given on the detection of IgM activity by ELISA also in other arboviral infections such as Japanese and LaCrosse encephalitis. The demonstration of an intrathecal production of virus-specific immunoglobulins may reveal the type of virus causing the infection in the central nervous system.