The monoclonal antibody is directed against the CD22- antigen, which is expressed on human B lymphocytes. Normal T-cells, polymorph nuclear cells, monocytes and platelets were found to be negative. Cell lines SB, Raji and NALM-1 were found to be positive. Most CALLA-ALL were found to be positive. The reaction pattern to CLL and NHL is variable from weak to negative. PLL is clearly positive and Hairy Cell Leukaemia is very strong positive with this antibody. Reacts with the 130-140 kD glycosylated type I integral membrane protein present on the surface of mature B cells. CD22 is expressed in the cytoplasm of virtually all B cells except plasma cells. CD45RO antigen on T cells and CD75 antigen on B cells have been identified as ligands for CD22. CD22 has been reported to participate in B-cell activation and also as an adhesion molecule. Although the immunobiology of this antigen has not been fully elucidated, reports indicate that ligation of CD22 induces constitutive internalization of the molecule followed by complete degradation.