Stimulation with TPO (100 ng/mL) results in the upregulation of α_IIbβ₃-mediated adhesion of ⁵¹Cr-labeled primary megakaryocytic cells and HEL cells to immobilized fibrinogen (A) or fibronectin (B). Binding of cells stimulated with TPO to various immobilized substrates was assessed as described in Materials and Methods. Results are expressed in cpm and represent means ± SD of five separate experiments.