Heat-stable antigen (HSA/mouse CD24) is expressed in both haematopoietic and neural cells. The small core protein of the molecule is extensively glycosylated and anchored to the membrane via glycosylphosphatidylinositol. The role of HSA in the developing brain as well as its functional properties are poorly understood. Here we show that the brain HSA is associated with N- and O-linked oligosaccharide moieties and decorated with the HNK-1 sulfated carbohydrate epitope. It can bind P-selectin but not E-selectin and this interaction requires divalent cations and is sensitive to high salt. Brain derived HSA is also capable of binding to the L1 adhesion molecule. This interaction is distinct from the P-selectin binding as it is resistant to high salt and does not require bivalent cations. Treatment of HSA with OSGE significantly reduced binding of both P-selectin and I.1. Our data suggest that HSA can bind P-selectin and I.1 by distinct mechanism and that the binding epitopes on HSA are in close proximity.