The invention relates to novel benzodiazepine derivatives with antiproliferative activity and more specifically to novel benzodiazepine compounds, such as those in formulas (V)-(VII). The invention also provides conjugates of the benzodiazepine compounds linked to a cell-binding agent. The invention further provides compositions and methods useful for inhibiting abnormal cell growth or treating a proliferative disorder in a mammal using the compounds or conjugates of the invention. The present invention is further directed to methods of preparing a conjugate of a cell-binding agent and a cytotoxic compound. The methods comprise the use of an imine reactive compound to enable efficient conjugations of cytotoxic compounds with cell binding agents.