Disclosed is a process for preparing an antibody-cytotoxic agent conjugate comprising the steps of: (a) contacting an antibody with a bifunctional crosslinking reagent to covalently attach a linker to the antibody and thereby prepare a first mixture comprising antibodies having linkers bound thereto, (b) subjecting the first mixture to adsorptive chromatography and thereby prepare a purified first mixture of antibodies having linkers bound thereto, (c) conjugating a cytotoxic agent to the antibodies having linkers bound thereto in the purified first mixture by reacting the antibodies having linkers bound thereto with a cytotoxic agent in a solution having a pH of about 4 to about 9 to prepare a second mixture comprising (i) antibody chemically coupled through the linker to the cytotoxic agent, (ii) free cytotoxic agent, and (iii) reaction by-products, and (d) subjecting the second mixture to adsorptive chromatography to purify the antibodies chemically coupled through the linkers to the cytotoxic agent from the other components of the second mixture and thereby prepare a purified second mixture of antibodies chemically coupled through the linkers to the cytotoxic agent.
