Highly purified myelin P0 glycoprotein was solubilized to 1-8 mg/ml in 0.1% sodium dodecyl sulfate (SDS), and the solution structure of the P0 assembly was studied using synchrotron x-ray scattering. The full-length P0, which was isolated from bovine intradural roots, included both the extracellular and cytoplasmic domains of the molecule. At the higher concentrations (4, 6, and 8 mg/ml, respectively), an x-ray intensity maximum was observed at 316 A, 245 A, and 240 A Bragg spacing. Because the position of this intensity depended on P0 concentration, it is most likely due to interparticle interference. By contrast, the position of a second intensity maximum, which was at approximately 40 A Bragg spacing, was invariant with P0 concentration. This latter intensity was accounted for by monodispersed, 80 A-diameter particles that are composed of eight, approximately 30 A-diameter spheres. Chemical parameters suggest that the 80 A particles correspond to the size of a tetramer of P0 molecules. Therefore, the approximately 30 A spheres would correspond to the sizes of the extracellular and cytoplasmic domains for each of the P0 monomers. The invariance of the second intensity maximum with P0 concentration indicates that the structure of the 80 A-diameter, tetrameric particles is unaltered. According to the liquid model for interparticle interference from charged spheres, the 80 A-diameter particle has 10 negative surface charges which likely arise from negatively charged SDS molecules bound to the transmembrane domain of P0. This binding, however, apparently does not alter the tetrameric assembly of P0, suggesting that intermolecular interactions involving extracellular domains and cytoplasmic domains likely stabilize this assembly. Some of our results have been published in abstract form (Inouye, H., H. Tsuruta, D. A. Kirschner, J. Sedzik, and K. Uyemura. Abstracts of the 4th International School and Symposium on Synchrotron Radiation in Natural Science, June 15-20, 1998. Ustron-Jaszowiec, Poland. p. 31).