Although the aetiology of multiple sclerosis (MS) has not been established, circumstantial evidence points to the involvement of both cell-mediated and humoral immune responses in the formation of demyelinating lesions. In view of the current controversy regarding the reactivity of T lymphocytes from MS patients to myelin basic protein (MBP), we reassessed T cell reactivity in MS using a sensitive and specific indicator of cell-mediated immune response. No significant difference in the reactivity to MBP was observed between MS patients and healthy subjects. Interestingly, significant reactivity to MBP was detected in the control group comprising patients with other diseases. Nevertheless, our demonstration that polymorphism in T cell receptor (TcR) alpha chain are related to MS and that, in demyelinating lesions, TcR usage is limited, suggests that T cells may recognise particular epitopes of a critical antigen involved in this disease. The search for a specific MS antigen recognised by the intrathecally synthesized immunoglobulins typical of MS has so far been unsuccessful. However, recent work, which has focused more particularly onto myelin components with externally located epitopes accessible to the immune response, appears to be more promising. One such antigen, myelin-oligodendrocyte glycoprotein (MOG), is clearly a target for immune attack. Indeed, highly specific antibodies to MOG have been shown to cause demyelination not only in vivo but also in vitro, as demonstrated by our study of the demyelinating effects of a monoclonal anti-MOG antibody on aggregating fetal rat brain cell cultures.(ABSTRACT TRUNCATED AT 250 WORDS)