Myelin P2 is a 14,800-Da cytosolic protein found in rabbit sciatic nerves. It belongs to a family of fatty acid binding proteins and shows a 72% amino acid sequence similarity to aP2/422, the adipocyte lipid binding protein, a 58% sequence similarity to rat heart fatty acid binding protein, and a 40% sequence similarity to cellular retinoic acid binding protein. In order to isolate cDNA clones representing P2, a cDNA library was constructed from poly(A+) RNA isolated from sciatic nerves of 10-day-old rabbit pups. By use of a mixed synthetic oligonucleotide probe based on the rabbit P2 amino sequence, 12 cDNA clones were selected from about 25,000 recombinants. Four of these were further characterized. They contained an open reading frame, which when translated, agreed at 128 out of 131 residues with the known rabbit P2 amino acid sequence. These cDNAs recognize a 1.9-kilobase mRNA present in sciatic nerve, spinal cord, and brain, but not present in liver or heart. The levels of P2 mRNA parallel myelin formation in sciatic nerve and spinal cord with maximal amounts being detected at about 15 postnatal days. This initial study will allow characterization of the P2 gene and its regulation, as well as further studies into the role of P2, the first metabolically active myelin-specific protein to be characterized at the genetic level.