The denaturation behaviour of calmodulin (CaM) in sodium n-dodecyl sulphate (SDS), dodecyl trimethyl ammonium bromide (DTAB), guanidine hydrochloride (GuHCl) and urea was studied by fluorescence spectrophotometry at 25 and 37-degrees-C in Tris-HCl buffer, pH 7.4. The sigmoidal denaturation curve was plotted in order to estimate the thermodynamic parameters, assuming a two-state mechanism in terms of the Pace model. SDS and DTAB, anionic and cationic surfactants, affect CaM on a millimolar level as a result of direct interaction between CaM and surfactant as an amphipatic molecule. GuHCl and urea affect CaM on a molar level as a result of indirect interaction with the surroundings of CaM (a change in the water structure). The thermodynamic data indicate a slight interaction in the case of SDS which induced incomplete unfolding of CaM. With DTAB, GuHCl and urea, unfolding of CaM took place to a much greater extent.