A capture-ELISA for detecting IgM in CSF was developed for the purpose of rapid and early diagnosis of herpes simplex virus (HSV) encephalitis. In 37 cases with sporadic HSV encephalitis the CSF IgM was measured. In 13 cases the results were positive, in the others it was negative. 17 cases with other central nervous system disorders were taken as control group in which only one case with Guillain-Barré syndrome showed positive test and all the others were HSV antibody negative except one with IgG positive in CSF who had undergone a brain operation two weeks prior to the test.