Recombinant monoclonal antibodies are beginning to revolutionize cancer therapy. In combination with standard chemotherapy, high response rates have been reported with antibodies of the human IgG1 isotype for treatment of non-Hodgkin's lymphoma and breast cancer. It is becoming apparent that targets for antibody-based therapies do not necessarily need to be absent from normal tissues but can be present there either in low copy numbers or with binding epitopes shielded from the therapeutic antibody. Here, we studied whether claudin proteins that form tight junctions in normal epithelia are still expressed on carcinoma cells and whether their extracellular domains can be recognized by antibodies. We show that mRNAs of claudins 1, 3, 4, and 7 are all expressed in different human carcinoma cell lines, while claudin 8 was selectively expressed in breast and pancreas cancer lines. Chicken polyclonal antibodies were raised against peptides contained within predicted extracellular domains of claudins 1, 3, and 4. Affinity-purified IgG fractions for claudins 3 and 4 were monospecific and bound to human breast and colon carcinoma lines, but not to a line of monocytic origin. Claudin 3 antibodies also homogeneously stained human renal cell carcinoma tissue and micrometastatic tumor cells as identified by cytokeratin staining in bone marrow biopsies of breast cancer patients. Fluorescence-activated cell sorting and immunocytochemistry indicated that claudin antibodies bound to the surface of tumor cells. By analogy to other tumor-associated antigens that are differentially accessible to antibodies on tumor vs normal tissue, we propose that certain claudin proteins have potential as targets for novel antibody-based therapies of carcinomas.