We have expressed two proteins that correspond to the transmembrane and cytoplasmic domains of integrin αIIb and β3 subunits. Characterization of these proteins, dispersed in anionic and zwitterionic micelles, revealed that, rather than interacting with each other, the two proteins associated into homodimers and homotrimers respectively. Moreover, studies using the TOXCAT assay system confirmed that the αIIb and β3 transmembrane domains can self-associate in biological cell membranes. Transmembrane domain-mediated homo-oligomerization provides a plausible structural basis for integrin clustering and could promote integrin activation as well. Indeed, replacing specific residues in the transmembrane helix of either αIIb or β3 with an asparagine residue resulted in a facilitated homo-oligomerization of the mutated transmembrane helix, promoted the formation of integrin clusters on the cell surface and shifted αIIbβ3 to its activated state. Thus these studies support the hypothesis that the transmembrane domains play a vital role in the function and regulation of αIIbβ3.