Whereas L26 (anti-CD20) is well established as a B-cell marker of high specificity for use in paraffin-embedded tissues and JCB117 (anti-CD79a) is increasingly used, a comparable additional pan-B-cell antibody has hitherto not yet been identified. Here we have studied the use of a novel anti-pan-B-cell marker Pax-5 for use in diagnostic pathology. Pax-5 encodes for BSAP (Pax-5), a B-cell-specific transcription factor, the expression of which is detectable as early as the pro-B-cell stage and subsequently in all further stages of B-cell development until the plasma cell stage where it is downregulated. Pax-5 is essential for B-lineage commitment in the fetal liver, whereas in adult bone marrow this transcription factor is required for progression of B-cell development beyond the early pro-B (pre-BI) cell stage. Among the B-cell genes that are present in early B-cell development and are upregulated by Pax-5 are CD19 and Igalpha (CD79a). We have tested a commercially available anti-Pax-5 antibody (anti-BSAP, clone 24) in a series of 592 routinely fixed and paraffin wax-embedded biopsies, including lymph nodes, bone marrow, and various other organs containing lymphoid tissues. Pax-5 protein (BSAP) was detected in all cases of precursor and mature B-cell non-Hodgkin lymphomas/leukemias. In addition, in 97% of classic Hodgkin lymphomas, Reed-Sternberg cells expressed Pax-5. However, Pax-5 was not detected in any of the multiple myelomas, solitary plasmacytomas, and 4% of diffuse large B-cell lymphomas. Among those diffuse large B-cell lymphomas not expressing Pax-5 were only those with terminal B-cell differentiation. All T-cell non-Hodgkin lymphomas, including ALCL and lymphoblastic lymphomas and leukemias, were negative. There was a strong association between Pax-5 and CD20 expression. We conclude that anti-Pax-5 is an excellent pan-B and pan-pre-B-cell marker. We have found that anti-Pax-5 is superior to anti-CD20 in the diagnosis of pre-B acute lymphoblastic leukemia and classic Hodgkin lymphoma versus ALCL of T and "null" cell type. It was also useful in differential diagnosis between lymphoplasmacytic lymphoma and plasmacytoma. Even though there is an excellent correlation between CD20 and Pax-5 expression, anti-Pax-5 exceeds the specificity and sensitivity of L26 (anti-CD20) because of its earlier expression in B-cell differentiation and its ability to detect all committed B cells, including classic Hodgkin lymphoma.