Denaturants activate several multimeric enzymes in reverse micelles [Garza-Ramos, G., Darszon, A., Tuena de G6mez-Puyou, M. & G6mez-Puyou, A. (1992) Eul: J. Biochem. 205, 509-5171. Here, the effect on activity and intrinsic fluorescence of pig heart lactate dehydrogenase (LDH) in reverse micelles [formed with 0.2 M cetyltrimethylammonium bromide in octanehexanol (8.6: 1, by vol.)] was explored at various water and guanidine hydrochloride (GdnMC1) concentrations. Emission fluorescence spectra of LDH in aqueous media and in micelles were similar. As in all aqueous media, 1.0 M Gdn/HCl in the water phase of reverse micelles produced fluorescence quenching and a blue shift of the maximal emission. In 5.0 M Gdn/HCl, instead of the red shift and significant quenching seen in water, the maximum emission further shifted to the blue and was only slightly quenched. Gdn/HCl titrations of activity and fluorescence changes of LDH in micelles with different water contents showed that at Wo