This invention relates to the use of monoclonal and polyclonal antibodies that specifically bind to and become internalized by mesothelin-positive cells and also induce an immune effector activity such as antibody dependent cellular cytotoxicity. The antibodies are useful in specific delivery of pharmacologic agents to mesothelin expressing cells as well as eliciting an immune-effector activity particularly on tumor cells and precursors. The invention is also related to cells expressing the monoclonal antibodies, polyclonal antibodies, antibody derivatives, such as human, humanized, and chimeric monoclonal antibodies, antibody fragments, mammalian cells expressing the monoclonal antibodies, derivatives and fragments, and methods of treating cancer using the antibodies, derivatives and fragments.