Chronic lymphocytic leukemia (CLL) is classically defined by the proliferation and accumulation of monoclonal CD5 positive B cells "arrested" at an intermediate stage of differentiation. The CD23 antigen (the low affinity receptor for IgE (Fc epsilon RII) is a 45 kD membrane glycoprotein primarily expressed on surface IgM and IgD positive B cells. The aim of this paper is to establish the link between CD23 and CLL disease and to propose that the CD23 molecule is not simply a marker of CLL disease but may also be involved in the proliferation of the leukemic B cells.