A method of making antibodies to a selected part P of an immunoglobulin molecule ZP, Z comprising the non-selected part of ZP, P being the selected part of ZP which comprises a hidden epitope of the light chain, said method comprises tolerizing the source of antibodies, by administering to the source a compound containing the non-selected part Z or a part of Z and administering antibodies to the non-selected part Z or a part of Z; wherein these administered antibodies are specific to an Fc region of ZP; and immunizing the source of antibodies with part of ZP having the selected part P exposed; and wherein Z comprises non-hidden epitopes of said immunoglobulin molecule, and wherein said compound containing the non-selected part of said molecule comprises a whole immunoglobulin, and wherein immunizing the source of antibodies with part of ZP comprises immunizing with free light chain.