The DA strain of Theiler's virus causes a chronic progressive demyelination in mice following intracerebral inoculation. Virus was isolated from chronically infected mice, and then grown in cell culture, and the isolates were compared with the parent virus used for inoculation. No defective interfering particles or temperature-sensitive virus were recovered, and capsid proteins appeared identical by SDS-PAGE. One of three isolates had evidence of genomic mutation by Tl ribonuclease oligonucleotide fingerprinting. The significance of these findings with regard to the generation and maintenance of persistence and to adaptation to cell culture is discussed. Also of interest was the marked difference between the DA fingerprint and that of GD VII, a serologically related strain with different biological activity.