Raman and infrared spectroscopy have been simultaneously applied, for the first time, to the study of myelin membranes and their proteolipid protein (PLP) so as to obtain information on the secondary structure of proteins and the ordering of lipid chains. The vibrational spectra were recorded at physiological pH using a non-denaturing detergent (n-octyl-beta-D-glucopyranoside) in phosphate buffer. Neither the buffer nor the detergent interfere spectroscopically with the amide bands from proteins. The spectra reveal that the predominant secondary structure in the polypeptide backbone in myelin is the helix. The proteolipid protein was found to be more disordered than the polypeptide arrangement of the myelin membrane, as deduced from the relative intensities and halfwidths of characteristic infrared amide I bands. beta-form and turns are also present, the amount of these structures being higher in PLP. The study of the Raman spectra of vC-C and vC-H regions made it possible to obtain information on the lipid chain order.