4,4'-Dianilino-1,1'-binapthyl-5,5'-sulfonate (bis-ANS) and 1-anilinonaphthalene-8-sulfonate (ANS) are hydrophobic probes that are widely used in protein folding studies, using their capacity to bind to hydrophobic regions of partially unfolded proteins and in turn leading to an increase in fluorescence. Here we reveal a novel chaperone-like activity for bis-ANS, which acted as a highly effective inhibitor for the thermal- or chemical-induced aggregation of alcohol dehydrogenase, insulin or the whole cell extract of Escherichia coli, with ANS showing a much weaker effect. The studies to elucidate the mechanism underlying this activity show that bis-ANS is able to form stable soluble aggregates with the denaturing proteins and dramatically increase its fluorescence intensity,upon incubation with aggregation-prone proteins. Moreover, we found that bis-ANS is able to prevent the heat inactivation of citrate synthase. These observations suggest that bis-ANS is able to block the exposed hydrophobic surfaces to suppress protein aggregation, acting in a way similar to what small heat shock proteins (one sub-class of molecular chaperones) do. The data presented here, together with the report that bis-ANS was able to suppress the amyloid formation of the prion peptide [J. Biol. Chem. 279 (2004) 5346], suggest that this molecule may be used as a potential protein stabilizer in addition to its current application as a hydrophobic probe. (c) 2005 Published by Elsevier Inc.