Disclosed is a process for preparing an antibody maytansinoid conjugate comprising the steps of: (a) contacting an antibody with a maytansinoid to form a first mixture comprising the antibody and the maytansinoid, then contacting the first mixture with a bifunctional crosslinking reagent comprising a linker, in a solution having a pH of about 4 to about 9 to provide a second mixture comprising (i) the antibody maytansinoid conjugate, wherein the antibody is chemically coupled through the linker to the maytansinoid, (ii) free maytansinoid, and (iii) reaction by-products； and b) contacting the second mixture with the maytansinoid to form a third mixture； and then contacting the third mixture with the bifunctional crosslinking reagent at a pH of about 4 to about 9 to provide a fourth mixture
