The frequency of virus-specific T cells in the cerebrospinal fluid of a patient with viral infection of the brain and meninges was determined by using a single-T-cell cloning technique where a representative sampling of T cells was cloned from the cerebrospinal fluid of a patient with varicella zoster viral (VZV) meningoencephalitis. That the derived T-cell clones were in fact clonal was shown by demonstrating, on Southern blot analyses, unique rearrangements of the T-cell antigen-receptor beta-chain genes of each clone. Five out of the 15 of the T4+ (CD4), 0/4 of the T8+ (CD8), and 0/1 of the T4+T8+ T-cell clones proliferated to VZV, while no clones proliferated to mumps virus or myelin basic protein. There was no clonal expansion of any VZV-reactive T cell in this patient's cerebrospinal fluid. As VZV meningoencephalitis is thought to be due to the reactivation of a dormant herpes zoster viral infection, it can be regarded as a secondary immune response. The presence of different T-cell receptor beta-chain gene rearrangements in each T-cell clone suggests that the T-cell response was polyclonal. These results demonstrate that a high frequency of polyclonal, T4+ antigen-specific T cells can be found in a naturally occurring, localized, immune response.