In view of the active role played by infiltrating cells in the pathogenesis of the multiple sclerosis (MS) lesion and its experimental analogs, immune system products have been suspected as participants in the observed myelin damage in these diseases. Tumor necrosis factor (TNF) is one member of a growing family of cytokines currently implicated in several disease processes in which immune cytolytic mechanisms are operative. This study examines the effect of a number of cytokines upon myelin in vitro. Recombinant human TNF, gamma-interferon (IFN-.) I), and interleukin-2 (IL-2), as well as T-cell supernatants, were all compared for myelinolytic effect in myehated cultures of mouse spinal cord. In addition, the patterns were compared with antiserum-induced (antigalactocerebroside) demyelination.'Although cultures exposed to normal nutrient medium displayed regularly myelinated fibers (FIG. l), those exposed to TNF (1 X lo'to 1 X lo4 U/ml) displayed a type of myelin dilatation unique to this system (FIG. 2) which appeared to result from the influx of water into the internodal periaxonal space (FIG. 3). The effect of TNF upon myelin had a delayed onset (18-24 hours after exposure) and was accompanied by the selective necrosis of oligodendrocytes. Neurons were not affected, but astrocytes became hypertrophic and participated in the phagocytosis of oligodendroglial debris. Some myelinated fibers progressed to demyelination by 72 hours of exposure. The myelin lesion was not reversed by returning the cultures to normal feeding medium for 3 days. In contrast to TNF, IFN-7 and IL-2 had little or no effect on myelinated CNS cultures. T-cell supernatants induced some myelin fragmentation over 72 hours but no dilatation of the type produced by TNF. The pattern of myelin disintegration induced by antigalactocerebroside antiserum differed markedly from the TNF picture in that by 4 hours of exposure, total lysis of myelin with little or no bubbling was achieved with the lytic process beginning in the outer layers of the sheath. We suggest that the myelin changes resulting from exposure to TNF are related to a selective activity upon ion channels on the surface of the axon or the oligodendrocyte, and this leads to water influx into the periaxonal space. Thus, TNF produces a physiologic demyelination (not structural, as is the case with antiserum) initially