This antibody reacts with the CD10- antigen (CALL-antigen), which is expressed on human lymphoblasts. The antibody reacts with early B lymphocytes (stem-cell, pre-B) and with the stem-cell of the lymphocyte lineage and immature thymocytes. Lymphoblasts of a patient with an Acute Lymphocytic Leukaemia of the c-ALL type were found to be positive. Normal B- and T lymphocytes, monocytes and platelets were found to be negative.