We have used the technique of antibody mediated cell dependent immune lysis to examine paired samples of serum and CSF for antibody to herpes simplex virus. The 40 patients studied had no inflammatory disease of the nervous system, yet 20 of the CSF specimens did have antiviral antibody. This is an extremely sensitive technique for the detection of at least one type of antiviral antibody and "in vitro" is a very effective way of killing virus infected cells. There is no correlation between the level of antiviral antibody in the CSF with the total protein content, but the high CSF: serum antibody ratio in some subjects who are particularly susceptible to recurrent herpes infection raises the possibility that local stimulation and production of this antibody may occur.