In this chapter, we describe a method for detecting the ubiquitination status of G protein-coupled receptors (GPCRs). This involves co-expression of a GPCR with an epitope-tagged ubiquitin construct in a -heterologous mammalian expression system. Stimulus-dependent modification of the GPCR by -ubiquitin is detected by immunoprecipitation and subsequent immunoblotting to detect incorporation of the epitope-tagged ubiquitin. We describe here a well-established protocol to detect ubiquitination of the chemokine receptor CXCR4, which can be easily applied to detect the ubiquitination status of other GPCRs.