The invention provides processes for manufacturing cell-binding agent-cytotoxic agent conjugates of improved homogeneity comprising performing the modification reaction at a lower temperature. The inventive processes comprise contacting a cell-binding agent with a bifunctional crosslinking reagent at a temperature of about 15° C. or less to covalently attach a linker to the cell-binding agent and thereby prepare a mixture comprising cell-binding agents having linkers bound thereto.
