Organotypic dorsal root ganglion (DRG) cultures were established from all the embryos of two trembler (Tr/+) female mice mated to normal (+/+) males to determine if the trembler mutation would be expressed in nerve tissue culture. Dorsal root ganglia from normal mice maintained in our culture system exhibit substantial myelination after 6 weeks of growth. This normal pattern was observed in approximately one half of the cultures in the present series. The remaining half of the explants had marked PNS myelin abnormalities readily detectable at the light microscopic level in living cultures; furthermore, the ultrastructural appearance of these Tr/+ cultures was similar to that of adult trembler sciatic nerve. An analysis of unmyelinated nerve fibers in Tr/+ cultures revealed that the number of neurites resident within each non-myelinating Tr/+ Schwann cell was significantly less than the number observed in +/+ cultures. There are distinctive PNS myelin abnormalities which: (1) develop in DRG cultures established from embryos at risk for the trembler mutation; (2) are highly reliable and readily detectable markers for the trembler genotype; and (3) are similar to the trembler PNS defects detectable in vivo.