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Retroviral involvement in the pathogenic cascade in multiple sclerosis (MS) and a cytotoxic activity with narrow specificity towards glial cells have been recently considered as credible working hypotheses to explain some of the complex pathophysiological and neuropathological features of MS. The partial characterization of exogenous retroviral sequences, thought to be associated with MS, has led us to the identification of new human endogenous retroviruses closely related to the extracellular multiple sclerosis associated retrovirus (MSRV). These endogenous retroviruses (HERV-TcR and HERV-7q) have the potential to be transcribed into RNA and proteins. Interestingly, the env domain of HERV-7q could code for a 59.8 kDa secreted glycoprotein (called enverin) with an immunoregulatory region. The presence in various MS biological fluids of a cytotoxic activity able to induce programmed cell death for oligodendrocytes and astrocytes suggests the possibility of a demyelination phenomenon as part of direct glial cell damage. Moreover, both retroviral expression and cytotoxic factor production have been evidenced in MS monocyte/macrophage cultures and MS cerebrospinal fluid. It is now crucial to better characterize the endo/exo retroviruses possibly involved in MS and their pathogenic potential, and to identify the contributing factor(s) to the gliotoxicity found in the MS cerebrospinal fluid or serum, as well as to elucidate the mechanism of induction of the observed programmed glial cell death.