titative measure for comparing the 'activity' of various of physiological interest. In such cases, lipid analogues wild-type and mutant forms. For this purpose, binding containing structure-perturbing anthroylxoxy or doxyl assays that are quantitative, accurate and robust are de­ probes are required. Lipids that do have intrinsic fluor­ sirable. escence, such as parinaric acid, are labile and prone to For the intracellular fatty acid- and lipid-binding pro­ oxidation. The binding of native ligands can be moni­ teins, a variety of biochemical and biophysical binding tored by isotope-directed NMR techniques, provided assays have been used. The biochemical assays include that enrichment with 13C or another suitable isotope is those based on gel-filtration [1-3], equilibrium dialysis feasible. Although such NMR methods are useful for de­ [4], Lipidex [5,6] and liposomes [7].