It has been previously reported that, during denaturation of creatine kinase by guanidinium chloride (GdmCl) or urea [Tsou (1986), Trends Biochem. Sci. 11, 427-429], inactivation occurs before noticeable conformational change can be detected, and it is suggested that the conformation at the active site is more easily perturbed and hence more flexible than the molecule as a whole. In this study, the thiol and amino groups at or near the active site of creatine kinase are labelled with o-phthalaldehyde to form a fluorescent probe. Both the emission intensity and anisotropy decrease during denaturation indicating exposure of this probe and increased mobility of the active site. The above conformational changes take place together with enzyme inactivation at lower GdmCl concentrations than required to bring about intrinsic fluorescence changes of the enzyme. At the same GdmCl concentration, the rate of exposure of the probe is comparable with that of inactivation and is several orders of magnitude faster than that for the unfolding of the molecule as a whole.