A membrane protein, OmpA of Escherichia coli, in the process of refolding from its heat-modified form in the presence of sodium dodecyl sulfate (SDS) to its non-heated one by the addition of systematic amounts of octylglucoside (OG) was characterized by means of dynamic light scattering and the size exclusion chromatography combined with low angle laser light scattering photometry. Upon heating in the presence of SDS only, the amount of SDS bound to OmpA was increased from 1.8 to 2.3 g/g of protein and its hydrodynamic radius increased from 3.7 to 4.7 nm. On the addition of OG, the once denatured OmpA regained its original size above the weight fraction of OG in the total amount of surfactants, 0.8. During the process, the hydrodynamic radius was observed to decrease cooperatively at the weight fraction of 0.6, while no change took place in the molar mass of the protein. The refractive index increment of OmpA reflecting the amount of surfactant binding also regained the value before the heating in parallel with the change of size. Examination of the amount of surfactants bound to the membrane protein according to known properties of the binary surfactant micellar system of the surfactants showed that SDS was principally responsible for the denaturing phenomena of OmpA. Copyright (C) 1998 Elsevier Science B.V.