In vivo, the surface glycoprotein Schwann cell myelin protein (SMP) is expressed in the quail peripheral nervous system exclusively by Schwann cells. It is not detectable at any developmental stage either in enteric glia or in ganglionic satellite cells. We demonstrate here that the satellite glial cells of the dorsal root ganglia start to express SMP on their surface when they are dissociated into single cells and cultivated in vitro. Activation of SMP synthesis is a rapid event observed in mass cultures of dorsal root ganglia dissociated cells as soon as 4 h after the onset of the culture. Confocal microscope analysis revealed that satellite cells may acquire the Schwann cell marker when still in close contact with the neuronal soma. Clonal cultures of satellite cells from E8 dorsal root ganglia demonstrated that the progeny of these SMP-negative cells steadily express SMP. This, together with similar results previously obtained with enteric glia, suggests that the SMP-positive phenotype is a constitutive trait of the peripheral glial cell lineage which is inhibited in satellite cells in vivo by the microenvironment prevailing in the peripheral nervous system ganglia.