The involvement of the proinflammatory cytokines tumour necrosis factor-α (TNF-α) and lymphotoxin (LT) in multiple sclerosis is suggested by the parallel occurrence of these proinflammatory cytokines in acute and chronic active multiple sclerosis brain lesions. We describe the use of in situ hybridization with radiolabelled cDNA oligonucleotide probes to detect and enumerate TNF-α and LT mRNA expressing mononuclear cells without culture, and after culture in the presence of myelin basic protein (MBP), control antigens or without antigen. Compared with patients with aseptic meningo-encephalitis, non-inflammatory neurological diseases and healthy controls, the multiple sclerosis patients had elevated numbers of TNF-α and LT mRNA expressing mononuclear cells in blood when enumerated without previous culture, and also after culture with MBP. The MBP-induced upregulation of TNF-α and LT was major histocompatibility complex (MHC) class II molecule dependent. Tumour necrosis factor-α mRNA expressing mononuclear cells were further enriched in the multiple sclerosis patients' CSF. Positive correlations were observed in multiple sclerosis between TNF-α and LT mRNA expressing blood mononuclear cells, MBP-reactive TNF-a and LT mRNA expressing cells, and TNF-α and interferon-γ (IFN-γ) mRNA expressing mononuclear cells. Upregulation of TNF-α correlated positively with exacerbation, enhanced disability and the secondary progressive phase of multiple sclerosis. Patients with optic neuritis, in many instances representing very early multiple sclerosis, had TNF-α and LT positive blood mononuclear cells that were elevated to the same extent as patients with clinically definite multiple sclerosis. The findings support the hypothesis that TNF-α and LT play a harmful role in the development of multiple sclerosis and suggest that TNF-α could be useful as a disease activity marker in multiple sclerosis.