CD24, also known as heat stable antigen, is a phosphatidylinositol-linked cell surface glycoprotein that is expressed at all stages of B cell development and on most thymocytes.1-3 The absence of expression from mature T cells is closely associated with their maturation from CD4+CD8+CD24+ thymocytes to either CD4+CD8-CD24- or CD4-CD8+CD24- T cells. CD24 is also present on mouse granulocytes, monocytes, Langerhans cells and erythrocytes.4-6 Its ligand is P-selectin (CD62P).7 CD24 may play a role in regulation of B cell proliferation and differentiation, and it has been proposed that heat stable antigen promotes homotypic adhesion between B lymphocytes.1-3 Can be used to inhibit B-cell differentiation.