The Ig-binding properties of protein L from Peptostreptococcus magnus and protein G from Streptococcus have been successfully combined through the construction of a novel hybrid protein, consisting of a single Ig-binding domain from each protein. The biophysical and biochemical properties of this construct have been characterized through equilibrium and pre-equilibrium fluorescence spectroscopy, circular dichroism, isothermal titration calorimetry, affinity chromatography, and conformational stability studies using a chemical denaturant in order to examine the structure and availability of ligand binding sites in each domain. These studies show that despite the small size of the protein (Mw = 16.5 kDa) each domain behaves in an independent manner with respect to the binding characteristics of the same domain in isolation. (c) 2007 Elsevier Inc. All rights reserved.