The present invention provides a novel method for preparing a cell-binding agent cytotoxic agent conjugate. The method comprises the step of reacting a cell-binding agent with a cytotoxic agent or a cytotoxic agent-linker compound having a reactive group capable of forming a covalent bond with the cell-binding agent at a pH between 4 to 9 in the presence of a buffer solution with high ionic strength, wherein the cell-binding agent comprises a lysine ε-ΝH2 group that forms a covalent bond with the cytotoxic agent or the cytotoxic agent-linker compound having an amine-reactive group. The cell-binding agent-cytotoxic agent conjugates prepared according to the methods described herein are also included in the present invention.
