This antibody reacts with the CD19- antigen (B4-antigen), which is expressed on human B lymphocytes. The monoclonal antibody is B lineage-specific and reacts with early B-cell precursors, pre-pre-B-cells, pre-B- cells, B-cells, intermediate B-cells, mature B-cells and some plasmacytoid cells, also reacts with pre-B-cell- lines, B lymphoblastoid cell-lines and Burkitt cell- lines, and with 50% of myeloma cell-lines. Plasma cells were found to be negative.