A method of producing a humanized antibody, the method comprising: (a) synthesizing immunoglobulin heavy chain double stranded DNA fragment libraries comprising complementarity determining region fragment encoding libraries and framework fragment encoding libraries, wherein each complementarity determining region fragment library is derived from a template antibody and each framework fragment library is derived from a human framework pool obtained from functionally expressed human antibodies; (b) synthesizing immunoglobulin light chain double stranded DNA fragment libraries comprising complementarity determining region fragment encoding libraries and framework fragment encoding libraries, wherein each complementarity determining region fragment library is derived from the template antibody and each framework fragment library is derived from a human framework pool obtained from functionally expressed human antibodies; (c) assembling from the heavy chain fragment libraries by stepwise liquid phase ligation of heavy chain framework encoding fragments and complementarity determining region encoding fragments in the order of: framework1 - complementarity determining region1 - framework2 - complementarity determining region2 - framework3 - complementarity determining region3 to produce a humanized heavy chain variable domain encoding library; (d) assembling from the light chain fragment libraries by stepwise liquid phase ligation of light chain framework encoding fragments and complementarity determining region encoding fragments in the order of: framework1 - complementarity determining region1 - framework2 - complementarity determining region2 - framework3 - complementarity determining region3 to produce a humanized light chain variable domain encoding library; (e) cloning the assembled humanized heavy chain variable domain library and the assembled light chain variable domain library into an expression vector to create a humanization library, and when said antibody is a full length antibody, the expression vector comprises a nucleotide sequence encoding heavy chain framework 4 derived from a human heavy chain variable domain of a functionally expressed human antibody and a nucleotide sequence encoding framework 4 derived from a human light chain variable domain of a functionally expressed human antibody; (f) transfecting the humanization library into cells; (g) expressing humanized antibodies in the cells to create a humanized antibody library; (h) screening the humanized antibody library to determine the expression level of the humanized antibodies; and (i) screening the humanized antibody library to determine the affinity of the humanized antibodies for an antigen compared to the affinity of the template antibody to the antigen. (a) synthesizing immunoglobulin heavy chain double stranded DNA fragment libraries comprising complementarity determining region fragment encoding libraries and framework fragment encoding libraries, wherein each complementarity determining region fragment library is derived from a template antibody and each framework fragment library is derived from a human framework pool obtained from functionally expressed human antibodies; (b) synthesizing immunoglobulin light chain double stranded DNA fragment libraries comprising complementarity determining region fragment encoding libraries and framework fragment encoding libraries, wherein each complementarity determining region fragment library is derived from the template antibody and each framework fragment library is derived from a human framework pool obtained from functionally expressed human antibodies; (c) assembling from the heavy chain fragment libraries by stepwise liquid phase ligation of heavy chain framework encoding fragments and complementarity determining region encoding fragments in the order of: framework1 - complementarity determining region1 - framework2 - complementarity determining region2 - framework3 - complementarity determining region3 to produce a humanized heavy chain variable domain encoding library; (d) assembling from the light chain fragment libraries by stepwise liquid phase ligation of light chain framework encoding fragments and complementarity determining region encoding fragments in the order of: framework1 - complementarity determining region1 - framework2 - complementarity determining region2 - framework3 - complementarity determining region3 to produce a humanized light chain variable domain encoding library; (e) cloning the assembled humanized heavy chain variable domain library and the assembled light chain variable domain library into an expression vector to create a humanization library, and when said antibody is a full length antibody, the expression vector comprises a nucleotide sequence encoding heavy chain framework 4 derived from a human heavy chain variable domain of a functionally expressed human antibody and a nucleotide sequence encoding framework 4 derived from a human light chain variable domain of a functionally expressed human antibody; (f) transfecting the humanization library into cells; (g) expressing humanized antibodies in the cells to create a humanized antibody library; (h) screening the humanized antibody library to determine the expression level of the humanized antibodies; and (i) screening the humanized antibody library to determine the affinity of the humanized antibodies for an antigen compared to the affinity of the template antibody to the antigen.