AFTER lesions in the differentiated central nervous system (CNS) of higher vertebrates, interrupted fibre tracts do not regrow and elongate by more than an initial sprout of ∼ 1 mm (refs 1–3). Transplantations of pieces of peripheral nerves into various parts of the CNS demonstrate the widespread capability of CNS neurons to regenerate lesioned axons over long distances in a peripheral nerve environment (for example, see refs 2 and 3). CNS white matter^4–6, cultured oligodendrocytes (the myelin-producing cells of the CNS), and CNS myelin itself, are strong inhibitors of neuron growth in culture⁷, a property associated with defined myelin membrane proteins of relative molecular mass (Mr) 35,000 (NI-35) and 250,000 (NI-250)⁸. We have now intracerebrally applied the monoclonal antibody IN-1, which neutralizes the inhibitory effect of both these proteins⁹, to young rats by implanting antibody-producing tumours. In 2–6-week-old rats we made complete transections of the cortico-spinal tract, a major fibre tract of the spinal cord, the axons of which originate in the motor and sensory neocortex^10,11. Previous studies have shown a complete absence of cortico-spinal tract regeneration after the first postnatal week in rats¹², and in adult hamsters and cats^13,14. In IN-1-treated rats, massive sprouting occurred at the lesion site, and fine axons and fascicles could be observed up to 7–11 mm caudal to the lesion within 2–3 weeks. In control rats, a similar sprouting reaction occurred, but the maximal distance of elongation rarely exceeded 1 mm. These results demonstrate the capacity for CNS axons to regenerate and elongate within differentiated CNS tissue after the neutralization of myelin-associated neurite growth inhibitors.