CD9 is a well-defined component of the platelet plasma membrane and has a copy number almost equivalent to that of glycoprotein (GP) IIb-IIIa complexes, the aggregation receptor on platelets. It has an apparent molecular mass of 24 kD and is otherwise known as p24. Stimulation of p24 by monoclonal antibodies (MAb) induces platelet aggregation and granule release, involves FcγRII, and is mainly mediated through the stimulation of phospholipase C. In accordance with a signalling function, p24 has been reported to associate with small GTP-binding proteins and to GP IIb-IIIa complexes upon activation. We now report further evidence of a strong relationship between p24 and GP IIb-IIIa in platelets. Using the MAIPA (monoclonal antibody immobilization of platelet antigens) assay in the screening of human antibodies to platelet glycoproteins, we found that GP IIb-IIIa-antibody complexes were almost invariably associated with p24 in the harvested detergent-soluble fraction of platelet lysates. Thus, associated human antibodies were detected following the targeting of either GP IIb-IIIa or p24 by monospecific murine monoclonal antibodies (MAbs). This is a point to bear in mind when assessing for antibodies to p24 or GP IIb-IIIa in immune thrombocytopenias.