Intracerebral (i.c.) inoculation of parainfluenza 1 (6/94) virus into weanling Lewis rats produced meningitis, choroiditis, ependymitis, and a rare noninflammatory white matter degeneration (WMD). Adoptive transfer of immune spleen cells (ISC), 3 to 5 days after virus infection, significantly increased the incidence of WMD. Conversely, ISC given 2 days prior to virus infection not only protected rats from developing WMD, but also from developing meningitis, choroiditis, or ependymitis. Thus the temporal relationship of 6194 virus inoculation to the virus-specific immune response appears critical in determining whether WMD will develop in association with 6/94 virus infection. Although sensitized immunocytes may prevent pathologic changes if given prior to virus inoculation, such immunocytes enhance the production of WMD if given after such inoculation. These studies underline the dual role of the immune response in 6/94 virus infection, and indicate that 6/94 virus-induced WMD is, in part. immunemediated.