To better characterize the inflammatory response that occurs in the nervous system in multiple sclerosis (MS), T-cell receptor (TCR) gene expression was quantified from cerebrospinal fluid (CSF) cells of 21 patients with active disease. Unstimulated CSF cells expressed each of 22 different TCR beta chain variable region (VB) gene families in proportion to their expression in simultaneously sampled peripheral blood. When CSF cells from individuals with MS were expanded by in vitro culture in T-cell growth factodinterleukin 2 and 4-containing medium (TCGF/IL2/IL4), restricted numbers of VD genes were expressed. In many subjects, expanded CSF cells expressed predominantly VB2. In contrast to CSF, expansion of corresponding peripheral blood mononuclear cells (PBMC) in TCGF/IL2/IL4 resulted in persistent expression of all VD gene families. Within individuals, different Vp genes were overexpressed by PBMC compared with CSF cells. No effect of the HLA haplotype of the individual on CSF VB gene expression was observed. Expanded CSF cells retained their capacity to respond to mitogen stimulation, but the proliferative response to myelin basic protein (MBP) was not enhanced. Finally, freshly obtained CSF cells stimulated directly with MBP also expressed a limited number of VP genes, although these were generally different from patterns observed following stimulation with TCGF/IL2/IL4. Thus, restricted populations of T cells capable of responding to TCGF/IL2/IL4, presumably reflecting in vivo activated cells, are compartmentalized in the nervous system in MS.