CD24 is expressed in B cells until their terminal differentiation into plasma cells.1 It has been used widely as a marker to fractionate different B-lineage differentiation stages.10 CD24 has been found on a variety of other hematopoietic and neural cell types, and is now classified as a costimulatory molecule.2-4 The CD24/HSA molecule is composed mostly of carbohydrate, with a protein core of only 31-35 amino acids in the human and 27 amino acids in the mouse.1, 3 The molecular mass is variable, ranging from 38-70kD.3 CD24 is now included in the large group of phosphatidylinositol-anchored membrane proteins.1-4 Studies with purified CD24 applied to latex beads have demonstrated a self-binding property, suggesting that the molecule is capable of promoting homotypic aggregation, binding and cell adhesion.3 This is supported by earlier observations of the blockade of B cell lymphoblast aggregation with anti-CD24 monoclonal antibody.6 Recent studies of murine CD24 have assigned a role for CD24/HSA as a ligand for P-selectin, a calcium-dependent lectin.7