To establish an early reliable diagnostic test for herpes simplex virus (HSV) type 1 encephalitis (HSVE), we used isoelectric focusing (IEF) and an IEF-overlay technique with radiolabeled HSV glycoprotein B (gB) to study 7 serum and 12 cerebrospinal fluid (CSF) samples from 12 patients with presumed or biopsy-proved HSVE. Blood-brain barrier damage and increased intra-blood-brain barrier IgG synthesis were detected in 5 of the 7 patients with HSVE. CSF oligoclonal bands were found in 6 of 11 patients. Using an IEF-overlay technique, we detected anti-gB antibody in all serum (7 of 7) and in 10 of 12 CSF samples. Anti-gB antibody was found in 4 of 6 CSF specimens obtained within the first week of disease (days 3 to 5) and in all samples collected later in the disease. The pH range of anti-gB antibody activity was broad (4.5 to 9.5), indicating a heterogeneous immune response to HSV. A hematogenous origin of the CSF antibody was suggested because anti-gB antibody appeared in serum before matched CSF and because both serum and matched CSF had a similar antibody IEF pattern. Local production of anti-gB antibody was suggested in some cases because of a greater prominence of anti-gB antibody in CSF than in matched sera and because CSF oligoclonal bands had anti-gB antibody activity. In contrast, only one of 6 CSF samples from patients with multiple sclerosis had gB antibody activity; in this case, anti-gB antibody activity did not correspond in isoelectric point location to oligoclonal bands.(ABSTRACT TRUNCATED AT 250 WORDS)