An inhibitor of the aggregation of immunoglobulin chains is provided. The inhibitor may comprise or consist of a polypeptide which comprises or consists of (a) an amino acid sequence corresponding to the amino acid sequence of the FR1 region of an immunoglobulin light chain variable domain, or part thereof which includes amino acid residue 12, (b) an amino acid sequence corresponding to the amino acid sequence of the immunoglobulin-binding domain of bacterial superantigen Protein L, or part thereof, and/or (c) an amino acid sequence corresponding to the amino acid sequence of the immunoglobulin-binding domain of streptococcal protein G, or part thereof, or a variant, fusion or derivative thereof, or a fusion of a variant or derivative thereof which retains the ability of the parent polypeptide to inhibit aggregation of immunoglobulin chains, or domains thereof. Other versions of the inhibitor are also provided.