As the major transbilayer glycoprotein of peripheral myelin, P0 is believed to play a prominent role in the formation of this structure by Schwann cells. The amino acid sequence of the chicken P0 molecule, deduced from the nucleotide sequence of cDNA clones, is reported here. Comparison with the mammalian molecule reveals an extensive overall homology, thus underlining the importance both of the cytoplasmic and of the extracellular domains of this protein in the establishment and preservation of peripheral myelin structure. Unexpectedly, an avian P0 cDNA probe was found to hybridise with several mRNA species, present exclusively in peripheral nerve and in proportions that varied according to the developmental stage. The expression of these transcripts was detected significantly earlier than that of P0 mRNA in mammals.