An inhibition enzyme-linked immunosorbent assay (ELISA) for the detection of herpes simplex virus antigens in cerebrospinal fluid (CSF) has been developed. A Triton X-100 extract of herpes simplex virus type 1 (HSV-1) infected HEp-2 cells was used to coat wells of polyvinyl chloride plates. Rabbit anti-HSV-1 globulin served as the reference antibody and the CSF specimens were tested at a final dilution of 1 : 4. Positive results were obtained in CSF specimens from 11 18 (61%) neonates with HSV infection, 15 23 (65%) older individuals with HSV culture positive brain biopsies, and in 4 29 (14%) patients with culture negative brain biopsies. The assay was negative with CSF from 14 infants without HSV infections, from 30 patients with bacterial meningitis and 10 with cryptococcal meningitis. The test was positive in 10 21 patients within 10 days of onset, 11 14 within 11-20 days, and in 5 6 more than 20 days after onset of the herpetic infection. The overall sensitivity of the assay was 63% and the specificity was 95%. © 1983.