A process for preparing an antibody-maytansinoid comprising the steps of: (a) contacting an antibody with a reactive bifunctional crosslinking 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 tangential flow filtration, selective precipitation, adsorptive filtration, or a resin adsorptive chromatography and thereby prepare a first purified mixture of antibodies having linkers bound thereto, (c) conjugating a maytansinoid to the antibodies having linkers bound thereto in the first purified mixture by reacting the antibodies having linkers bound thereto with a maytansinoid in a solution having a pH of 4 to 9 to prepare a second mixture comprising (i) antibody chemically coupled through the linker to maitansino ide, (ii) free maytansonid and (iii) reaction by-products, and (d) subjecting the second mixture to a tangential flow filtration to purify the antibodies chemically coupled through the linkers to the maytansinoid from the other components of the second mixture and prepare thereby a second purified mixture of chemically coupled through the linkers to the maytansinoid antibody.Un procedimiento para preparar un conjugado anticuerpo-maitansinoide, que comprende las etapas de: (a) poner en contacto un anticuerpo con un reactivo de reticulación bifuncional para unir covalentemente un enlazador al anticuerpo y preparar de este modo una primera mezcla que comprende anticuerpos que tienen enlazadores unidos a los mismos, (b) someter la primera mezcla a filtración de flujo tangencial, precipitación selectiva, filtración adsortiva, o una resina de cromatografía adsortiva y preparar de este modo una primera mezcla purificada de anticuerpos que tienen enlazadores unidos a los mismos, (c) conjugar un maitansinoide con los anticuerpos que tienen enlazadores unidos a los mismos en la primera mezcla purificada haciendo reaccionar los anticuerpos que tienen enl
