Rabbit antisera to the rat myelin proteins P0 and P1 were used to assay for the presence of these components by both immunochemical and immunofluorescence methods. The antiserum to P0 did not react detectably with polyacrylamide gels containing central myelin, or with P1 and P2 in peripheral myelin; it did react with P0 in peripheral myelin, and in extracts of adult and neonatal sciatic nerve. When reacted with frozen tissue sections using indirect immunofluorescence, it did not stain central myelin but did stain myelin in adult sciatic nerve, the myelinated fibres in cervical sympathetic trunk and occasional areas in neonatal sciatic nerve where Schwann cells had presumably begun to form myelin. Antiserum to basic protein reacted with both of the basic protein bands in central and peripheral myelin, but not P0; P1 and P2 were detectable in adult and neonatal sciatic nerve. In indirect immunofluorescence assays, the antiserum stained both central and peripheral myelin, the few myelinated fibres of sympathetic trunk and myelinating regions of neonatal sciatic nerve. Cultured secondary rat Schwann cells showed no detectable reaction with either reagent, using either technique. We conclude that these three proteins are probably expressed as a consequence of the neuron-Schwann cell interaction that initiates myelination.