The role of axons in the expression of the major myelin glycoprotein, PO, has been investigated using neuronkhwann cell cultures. These cultures were either nonmyelinating or myelinating due to growth in defined medium or in medium containing serum and chick embryo extract, respectively. The neurons and Schwann cells used in the studies were derived from embryonic day 15 rat dorsal root ganglia (DRG), and the Schwann cells from these ganglia are shown not to synthesize appreciable levels of PO prior to growth in culture. Myelinating cultures of Schwann cells and neurons grown together for 18-21 days synthesize PO that is readily identified by immunoblotting. The nonmyelinating cultures, which do not assemble basal lamina, also synthesize PO that is detectable by either ["Hlmannose precursor incorporation or by immunoblotting. The steady-state level of PO in the nonmyelinating cultures is less than that of the myelinating cultures, and the PO that is synthesized by the former appears to be catabolized shortly after its biosynthesis. Since nonmyelinating Schwann cells synthesize P,, when in contact with neurites in vitro, we have examined the ability of such nonmyelinating cells to express the glycoprotein in vivo. Very little steady-state PO is detected in immunoblots of the adult rat cervical sympathetic trunk (CST), a nerve in which approximately 99% of the axons are nonmyelinated. Similarly, the amounts of ["Hlmannose and ["Hlamino acids that are incorporated into newly synthesized PO are much lower in the CST than in the adult sciatic nerve. The Schwann cells of the CST are induced to synthesize myelin after growth in serum-containing medium with embryonic DRG neurites that are devoid of endogenous Schwann cells due to prior treatment with an antimitotic agent. Immunoblot analysis of these cultures reveals a level of PO that is comparable to that of myelinating DRG Schwann cellheuron cultures. These data indicate that DRG neurites supply a signal that can initiate the synthesis of PO in the absence of basal lamina and myelination. Furthermore, this signal may be reduced or altered in the CST.