Method for increasing the percentage of monomeric antibody Fab-dsFv multimeric species

The present invention provides method for increasing the percentage of monomer in a composition of recombinantly expressed antibody molecules characterised in that the antibody molecule comprises at least one Fv with specificity for an antigen of interest comprising one VH and one VL wherein said VH and VL are connected directly or indirectly via one or more linkers and are stabilised by a disulfide bond therebetween, said method comprises a) a thermal conversion step of holding the composition comprising the antibody molecule at a temperature in the range 30 to 60° C. for a period of at least 1 hour, wherein step a) is performed in the presence of a reducing agent or after treatment with a reducing agent.