In vitro assays reveal that myelin and oligodendrocytes of the Xenopus spinal cord (SC) are--unlike corresponding components of the optic nerve/tectum (OT)--nonpermissive substrates for regenerating retinal axons. The number of growth cones that crossed SC oligodendrocytes is low but increases significantly (four- to fivefold) in the presence of the antibody IN-1, in which case their numbers are similar to the number of growth cones (approximately 60%) that cross OT oligodendrocytes with or without IN-1. IN-1 neutralizes neurite growth inhibitors (NI) of rat CNS myelin, indicating that mammalian-like NI are associated with Xenopus SC myelin and oligodendrocytes but not with the OT. IN-1 immunocytochemistry on sections supports this view: SC myelin was stained with IN-1, whereas OT myelin and PNS myelin were not.