This invention relates to the discovery of a differentiation antigen termed mesothelin which is associated with mesotheliomas and ovarian cancers. Mesothelin is about 69 kD in its full-length form. The invention includes uses for the amino acid and nucleic acid sequences for mesothelin, recombinant cells expressing it, methods for targeting and/or inhibiting the growth of cells bearing mesothelin, methods for detecting the antigen and its expression level as an indication of the presence of tumor cells, and kits for such detection.