Protein L binds to the V_L domain of κ-chains enabling it to bind to all sub-classes of antibodies that contain κ-chains. The complex formed between a single domain of wild-type Protein L and κ-chain is described by a dissociation constant (Kd) of approximately 150nM. Such domains, used as affinity ligands for the purification of κ-chains, or Igs bearing κ-chains, require harsh elution conditions to dissociate the complexes. This leads to degradation of the affinity column and the Ig or Ig fragment being isolated. The aim of the work described in this thesis was to develop variants of a single domain of Protein L that can offer more favoured binding characteristics improving their usefulness in some commercial applications. Thus mutants have been developed that have lower affinities for κ-chain (and Ig bearing κ-chain) but retain satisfactory conformational stabilities. A number of mutated domains have been purified and their binding properties to κ-chain characterised by ELISA, competitive ELISA, affinity chromatography and equilibrium and pre-equilibrium fluorescence spectroscopy. These studies have shown that whereas the F39W, the N26,76D, the N26,76D,F39W, and the K33W mutants all have low dissociation constants for their complexes with κ-chain of Ig, two mutants Y53H,F39W and F39H,Y64W have substantially higher Kd values. Both of these proteins were immobilised onto Sepharose 4B gel and used in affinity chromatography experiments. These showed that human Ig was released from these two proteins at pH 3.9 and 3.8 respectively and thus make useful affinity ligands. Pre-equilibrium binding studies have also provided a further insight into the possible mechanism by which κ-chains bind Protein L giving rise to the possibility that there are in fact two binding sites for κ-chain on a single domain of Protein L.