[en] Class A beta-lactamases (260-280 amino acids; M ( r ) ~ 29,000) are among the largest proteins studied in term of their folding properties. They are composed of two structural domains: an all-alpha domain formed by five to eight helices and an alpha/beta domain consisting of a five-stranded antiparallel beta-sheet covered by three to four alpha-helices. The alpha domain (~150 residues) is made up of the central part of the polypeptide chain whereas the alpha/beta domain (111-135 residues) is constituted by the N- and C-termini of the protein. Our goal is to determine in which order the different secondary structure elements are formed during the folding of BS3. With this aim, we will use pulse-labelling hydrogen/deuterium exchange experiments, in combination with 2D-NMR measurements, to monitor the time-course of formation and stabilization of secondary structure elements. Here we report the backbone resonance assignments as the requirement for further hydrogen/deuterium exchange studies.