Binding of CD24-coated beads and KS cells to platelets and P-CHO. PMA-activated platelets were adsorbed to 96-well tissue culture plates at 37°C for 1 hour (A). P-CHO cells were grown to confluency in 96-well plates (B). Europium-labeled KS tumor cells were allowed to bind in the absence or presence of the indicated antibodies or 2 μg/mL soluble CD24 or HSA, respectively. Bound and nonbound cells were separated by buoyant density as described in Materials and Methods. After fixation of bound cells the Europium fluorescence was measured. (C) PMA-activated platelets were adsorbed to glass slides. CD24 beads and control beads were allowed to bind in the presence or absence of soluble glycoproteins or the indicated MoAbs. The binding assay was performed for 30 minutes at room temperature. The slides were washed and fixed in 2% glutaraldehyde before counting.