A cytotoxic agent comprising one or more taxanes linked to a cell binding agent. A therapeutic composition for killing selected cell populations comprising: (A) a cytotoxic amount of one or more taxanes covalently bonded to a cell binding agent through a linking group, and (B) a pharmaceutically acceptable carrier, diluent or excipient. A method for killing selected cell populations comprising contacting target cells or tissue containing target cells with an effective amount of a cytotoxic agent comprising one or more taxanes linked to a cell binding agent. Novel sulfur-containing taxanes.