This paper describes a new medium which promotes myelin formation in cultures of rat dorsal root ganglia, and controls fibroblast proliferation. The medium, medium 'M', contains D-valine rather than L-valine and is added to cultures after seven days in vitro (DIV). The presence of D-amino acid oxidase in Schwann cells and neurons permits these cells to convert D-valine to L-valine, an essential amino-acid. Fibroblasts however lack D-amino acid oxidase but as some L-valine is present in the fetal calf serum component of medium 'M' those fibroblasts already present in the cultures by day 7 are maintained. Further fibroblast proliferation is however inhibited. The use of medium 'M' does not require the use of specific antimitotic agents making it cheaper and quicker to obtain myelinated cultures than with previous methods. The medium results in widespread myelin formation by 21 DIV. Cultures may be grown on plastic or collagen coated glass coverslips.