Major outer membrane proteins of Escherichia coli K-12 with apparent molecular weights ranging from 30,000 to 40,000 were resolved into four distinct bands by electrophoresis on an improved urea-sodium dodecyl sulfate (SDS)-polyacrylamide gel containing a high concentration of N,N'-methylenebisacrylamide. The electrophoretic mobilities of three of these proteins, O-8, O-9, and O-10, changed when they were heated in SDS solution. Proteins O-8, O-9, and O-10, were purified to near homogeneity without heating in SDS solution. Electrophoretic profiles of the purified proteins changed depending on the solubilization temperature in SDS solution. Infrared and CD spectra of these proteins revealed that they were extremely rich in beta-structured polypeptide, which is stable in SDS solution at room temperature. On the other hand, CD spectra typical of alpha-helix structure were obtained when the proteins were heated in SDS solution, indicating that a gross conformational change occurred in the protein molecules upon heating in SDS solution. The conformational change was confirmed by the abnormal profiles of Ferguson plots in gel electrophoretic analysis. It was concluded that conformational changes in the protein molecules are responsible for the heat modifiability of these proteins in SDS gel electrophoresis.