Here we describe an approach to investigate di- or oligomerization of transmembrane receptors in living cells with fluorescence recovery after photobleaching (FRAP). We immobilized a defined fraction of receptors with antibodies and then measured lateral mobility of the nonimmobilized fraction by FRAP. We validated this approach with CD86 and CD28 as monomeric and dimeric reference proteins, respectively. Di- or oligomerization of G protein-coupled receptors is strongly debated. We studied human beta-adrenergic receptors as prototypical G protein-coupled receptors and found that beta(1)-AR shows transient interactions whereas beta(2)-AR can form stable oligomers. We propose that this FRAP method can be widely applied to study di- or oligomerization of cell-surface proteins.