CD19 recognizes a 95kD cell surface glycoprotein, which is expressed by cells of B-cell lineage and follicular dendritic cells. CD19 is an important signal transduction molecule in the regulation of B-lymphocyte development, activation and differentiation. Studies have shown that CD19 is absent in plasma cells, most T-cell lymphomas and in lymphocyte predominant Hodgkin’s. It has been observed in lymphomas and leukemias but often weak/negative in follicular lymphoma or diffuse large B-cell lymphoma. CD19 may provide useful diagnostic information for the study of B-lymphoproliferative disorders.