The secondary structure of myelin basic protein (MBP) in reconstituted central nervous system myelin was studied using Fourier transform infrared (FTIR) spectroscopy. The spectra of the protein in aqueous solution and in the lipid environment were compared and notable differences were observed. It is proposed that there are significant differences in the conformation of the protein in the contrasting environments. Significant increases in both alpha-helical structure and beta-structure were observed on reconstitution in myelin. The findings of this study also support the view that the presence of both alpha-helices and beta-structure plays a important role in membrane proteins.