It is now well established that plasma membranes, such as the myelin sheath, are made of different microdomains with different lipid and protein composition. Lipid rafts are made mainly of sphingolipids and cholesterol, whereas the non-raft regions are made mainly of phosphoglycerides. Most myelin proteins may distribute themselves in raft and non-raft microdomains but the driving force that gives rise to their different distribution is not known yet. In this paper, we have studied the distribution of protein zero (P0), the most representative protein of PNS myelin, in the membrane microdomains. To this end, we have purified P0 from both non-raft (soluble P0, P0-S) and raft (P0-R) regions of PNS. Purified proteins were analyzed by two-dimensional gel electrophoresis and identified and characterized by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. A detailed structural description of the two P0 forms is given in terms of amino acid sequence, post-translational modifications, and composition of associated lipids. Our findings suggest that structural differences between the two proteins, mainly related to the glycogroups, might be responsible for their different localization.