The invention provides a process for preparing an "antibody-maytansinoid" conjugate. The process comprises the steps of: (a) contacting an antibody with maytansinoid comprising a thiol group to form a first mixture comprising the antibody and 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 maytansinoid, (ii) free maytansinoid, and (iii) reaction by-products； and (b) purifying the second mixture comprising the "antibody-maytansinoid" conjugate to provide a purified "antibody-maytansinoid" conjugate. Also provided is a process for preparing an "antibody-maytansinoid" conjugate comprising the steps of: (a) contacting an antibody with maytansinoid comprising a thiol group to form a first mixture comprising the antibody and maytansinoid, then contacting the first mixture with a bifunctional crosslinking reagent comprising a linker, in a solution having a pH of 4 to 9 to provide a second mixture comprising (i) the "antibody-maytansinoid" conjugate, wherein the antibody is chemically coupled through the linker to maytansinoid, (ii) free maytansinoid, and (iii) reaction by-products； (b) contacting the second mixture with additional maytansinoid to form a third mixture； and then contacting the third mixture with an additional bifunctional crosslinking reagent at a pH of about 4 to about 9 to provide a fourth mixture； and c) purifying the fourth mixture comprising the "antibody-maytansinoid" conjugate to provide a purified "antibody-maytansinoid" conjugate.Изобретение описывает способ получения конъюгата "антитело-майтансиноид". Способ включает стадии: (а) приведение антитела и майтансиноида, включающего тиоловую группу, в контакт с образованием первой смеси, включающей антитело и майтансиноид, затем приведение первой смеси
