We analyzed the effects of myelin basic protein (MBP)-specific encephalitogenic T line cells in neonatal syngeneic recipients before and after formation of central nervous system myelin. Lewis rat pups (postnatal days 0, 1, 2, 4, 8) were injected intraperitoneally with MBP-specific T cell line in doses which cause clinical and histologic changes of experimental autoimmune encephalomyelitis (EAE) in adult rats. We correlated the susceptibility to transferred EAE with the developmental appearance of MBP as demonstrated by immunohistochemical staining. MBP was barely demonstrable until postnatal day 2 but became definitely apparent on postnatal day 4. All newborn rats that were injected on postnatal day 0, 1, 2 failed to develop any apparent clinical signs, but histologically some recipients displayed slight inflammatory cell infiltration of the meninges and subpial lesions limited to the lower spinal cord. In striking contrast, rats that were injected on postnatal day 4 or 8, developed clinical signs and, in particular, many of the older recipients became moribund. Histologically, these animals displayed marked inflammatory cell infiltrations and white matter destruction within the spinal cord. Clinical and histologic severity clearly increased with the recipient's age. Histologically, there were some differences between adult rat EAE and newborn rat EAE. In contrast to adult rat EAE which displayed grey and white matter involvement with marked perivascular mononuclear cell infiltrations, newborn rat EAE typically showed very severe changes with edema formation selectively in the white matter and the cellular infiltrates were dominated by polymorphonuclear cells and macrophages. Newborn T line-mediated EAE thus strikingly resembles hyperacute EAE induced in immunocompromised (irradiated) recipients.