Recombinant human tumor necrosis factor (rhTNF) has been tested for its effect on myelinated cultures of mouse spinal cord tissue. As controls, recombinant human interferon gamma (rhIFN) and interleukin-2 (rhIL-2) were tested, as well as T-cell supernatants, antigalactocerebroside serum, and normal culture medium. It was found that rhTNF induced delayed-onset (18-24 hr) oligodendrocyte necrosis and a type of myelin dilatation peculiar to this system. Some nerve fibers progressed to demyelination by 72 hours. The myelin dilatation was not reversible by return to normal feeding solution for 3 days. In contrast, rhIFN, rhIL-2, T-cell supernatants, and normal medium had little or no effect on cultures. This mechanism differs from other immune-mediated mechanisms in that it appears that a physiological (not structural) demyelination occurs initially without overt destruction of the myelin sheath. These observations are relevant to the evolution of the multiple sclerosis plaque: dysfunction of ionic channels might contribute to the eventual demise of oligodendrocytes and axons in the longstanding lesion.