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Abstract (summary): Much evidence indicates that G protein-coupled receptors can form oligomers, but the size and stability of those oligomers has not been well characterised. We used single-particle tracking (SPT) and Förster resonance energy transfer (FRET) to measure the oligomeric size of the M2 muscarinic receptor, a prototypical class A GPCR, in live cells. Single-particle intensity distributions that we obtained for the monomeric control CD86 and the dimeric control CD28 are nearly identical, and no conclusion about the oligomeric size of M2 receptors could be drawn from SPT measurements. FRET measurements allowed us to distinguish the monomeric control CD86 from the dimeric controls CD28 and caveolin-1, and the pattern of efficiencies produced by M2 receptors is similar to the pattern produced by the monomers but not the dimers. The view of M2 muscarinic receptors as transient oligomers appears to be the most consistent with other studies using different biochemical and biophysical techniques.