The interaction of the anionic surfactant sodium dodecyl sulphate (SDS) with water soluble protein Bovine Serum Albumin (BSA) has been investigated spectroscopically using fluorescence probe 1-hydroxy-2-naphthaldehyde (HN12). The characteristic intramolecular proton transfer fluorescence band of HN12 has been used as an efficient reporter for the study of binding of SDS with BSA. The changes of spectral properties of HN12 demonstrate that SDS plays two opposite roles in the stability of protein BSA. It acts as a stabilizer at low concentration and destabilizer at high concentration to urea-denatured BSA. (c) 2008 Elsevier B.V. All rights reserved.