The stability of three related subtilisin-like proteinases; a proteinase from a psychrotrophic Vibrio-species, proteinase K and aqualysin I from the thermophile Thermus aquaticus, to denaturation by GdmSCN, was measured in the presence and absence of selected lyotropic salts (sulfate and thiocyanate). How these salts affect protein stability has been ascribed to their effect on the strength of hydrophobic interactions. The salts affected the stability of the proteinases according to their ranking in the lyotropic salt series. The effect of the salts was smallest, however, on the stability of the cold-adapted Vibrio-proteinase, but largest for the thermophilic aqualysin I. These results suggests that the cold-adapted enzyme may be less dependent on hydrophobic interactions for structural stability than its counterparts adapted to higher temperatures.