Biopsies were obtained from 12 patients suspected of having nasopharyngeal carcinoma (NPC). A portion of the tissue was submitted for histopathology and another for Western blotting using a murine monoclonal antibody (MAb), MA6. Touch smears of the tissues were also prepared immediately prior to extraction and Western blotting for immunoenzymic staining. The results showed that a B-lymphocyte carcinoma cross-reacting antigen (BLCa), or an antigen similar to it, was the major antigen in the tumor tissue recognized by MA6. The antigen was detected in tissues from 8 patients, of whom 7 had confirmed NPC and one had eskimoma, but not in tissue from the remaining 4 patients who did not have histologically confirmed malignancy. Immunocytology showed that tumour cells were present in the touch smears from all but one of the tumour patients but not in the other patients, and that the tumour cells comprised the large majority of the MA6-reactive cell population. The other MA6-reactive cell types present included certain weakly reactive epithelial cells and occasional lymphoid cells, presumably B lymphocytes. However, these cell types were similarly distributed between the tissues obtained from patients with or without malignant diseases. It was concluded, therefore, that the tumour cells in these tissues are the principal source of BLCa and, as such, the antigen may constitute an objective and reliable marker of NPC.