We have raised a murine 1gM monoclonal antibody (MA6) against immunoprecipitate obtained by reacting the serum of an NPC patient with extract of the Burkitt lymphoma cell line, Raji. It reacts against an antigen (BLCa) which is broadly represented on human B lymphoid tissues and cell lines but is different from the functional B‐cell markers such as surface immunoglobulins, la products, Fc and complement receptors. BLCa was found to occur on lymphoid cell lines representing all stages of B‐cell differentiation. These included the pre‐B and null‐cell lines, Nalm 6 and Reh, the EBV‐transformed lymphoid cell lines and the myeloma cell line. MA6 was reactive against all the Burkitt lymphoma cell lines, whether or not these harbored the EBV genomes, with the exception of P3HR‐1. The antibody was found to be selectively reactive against a proportion of peripheral blood B lymphocytes and to stain the B‐cell‐rich primary follicles and mantle zones of secondary follicles in the lymph node. However, MA6 was not reactive against cell lines of T‐lymphocyte, myeloid, monocyte fibroblast or epithelial origin. It did, nevertheless selectively stain tumor cells from a variety of carcinoma tissues originating from different anatomical sites, including the nasopharynx.