1-Deoxy-1(N-hydroxyamino)-D-glucitol has been fixed onto human and bovine serumalbumin through a pentamethylene spacer arm. The modified proteins were purified by dialysis, electrophoresis and high pressure liquid chromatography to insure, in particular, a complete removal of low molecular weight starting materials. The fixation of the sugar derivative onto the protein was established by an increase in sugar content and the obtention of usable e.s.r. spectra from the nitroxide free radicals, obtained from the spontaneous oxidation of the N-hydroxyamino group. E.s. r. hyperfine coupling constants gave information on the conformation of the magnetic probe.