Lymphocytes from patients with transitional-cell carcinoma (TCC) of the urinary bladder were transformed by infection with Epstein-Barr virus. To obtain B cells secreting antibodies reactive with TCC cells, the transformed cells were either adhered to irradiated monolayers of cultured allogeneic TCC cells or subcultured at limiting dilution. Supernatants from these cultures were tested in a modified enzyme-linked immunosorbent assay against fixed cells, isolated plasma membranes, or lipid antigens or were tested by antibody-dependent cellular cytotoxicity (ADCC). Reactions with antigens derived from the serum source were excluded by proper controls. By this approach a majority of the patients tested (7/12) gave rise to cultures producing antibodies recognizing various cellular antigens. The antibody-containing supernatants from these cultures were usually of high titres and the reactive antibodies of IgM isotype. One culture, which had been selected by repeated adherence to TCC cells, produced antibodies reactive with such cells in ADCC. None of the antibodies investigated detected antigens exclusively present on TCC cells.