MAb IPO-10 defines the antigen, which appears on B cell progenitors following HLA-DR and preceding CD10, CD19, CD22, CD37 and cym. It is expressed on resting B cells and than reappears and persists in cytoplasm and on cell surface until cytoplasmic Ig appears. It is a useful antibody for diagnostics of neoplasms of B cell origins. It reacts with human B cell lines Daudi, Raji, Namalva, EB-3, RPMI-8226 (50% of cells). The MAb does not label T cell lines, blood granulocytes, thymocytes or bone marrow stromal fibroblasts. No significant changes are detected after PHA or ConA stimulation while LPS and PWM stimulated cultures after 18-48h show decreased number of antigen-positive cells but in final terms of cultivation antigen is expressed again. This MAb labels B cell leukemias and some lymphomas. Hairy cell leukemia strongly reacts and 70% of B cell CLL and some B-NHL were also positive. IPO-10 reacts with AMML cells and in a majority of Hodgkin’s disease cases a significant percentage of affected lymph node cells were detected.