Several cell adhesion molecules of the immunoglobulin superfamily (IGSF) are expressed differently during the development and myelination of the peripheral nervous system. To examine the relationship between the expression of IGSF molecules and Schwann cell differentiation, we established a useful system for myelin formation in vitro on collagen gel using primary neuron/Schwann cell cocultures from neonatal dorsal root ganglions (DRG). At 10 days in vitro (DIV), many Schwann cells were found in the areas surrounding aggregates of DRG neurons. After 20 DIV, Schwann cells posi-tioned next to axons and elongated their processes along the axons. Some of them started loosely elaborating a large axon. Under electron microscopy, compact myelin was shown to be formed at 30 DIV. Thus the speed of myelination was much slower in vitro than in vivo. In co-cultures, L1 and neural cell adhesion molecule (NCAM) were detected at the premyelinating stage, L1 was precisely expressed earlier than NCAM. Expression of myelin associated glycoprotein (MAG) was transiently up-regulated at the early stage of myelination, and then P0 expression was finally increased as myeli-nation proceeded. The change of expression pattern of these molecules in co-cultures was quite similar to that observed in the development in vivo. When Schwann cell proliferation was blocked by low serum culture condition, Ll and NCAM expressions were up-regulated. In contrast, the presence of cholera toxin in low serum media markedly increased expressions of P0 and MAG, but decreased the levels of both Ll and NCAM. These results suggest that both Ll and NCAM play roles in the contact and/or recognition between axons and Schwann cells at an early stage of myelination. On the other hand, MAC and P0 are important for axon ensheathment and myelin compaction.