Spontaneous demyelination has been observed to occur sporadically in the thoracic spinal cord of aging laboratory mice used in routine long-term chemical safety studies. Positive immunohistochemical reaction for VP-1 (virus protein) indicated that the demyelination was associated with Theiler's murine encephalomyelitis virus infection. Although this rare spontaneous lesion has no bearing on the quality of chemical safety studies, its knowledge is essential for the appropriate interpretation of study results.