The development of a multicellular organism is accompanied by cell differentiation. In fact, many biological processes have cell specificity, such that distinct cell types respond differently to endogenous or environmental cues. To obtain cell-specific gene expression profiles, translating ribosome affinity purification (TRAP) has been developed to label polysomes containing translating mRNAs in genetically defined cell types. Here, we describe the immunopurification of epitope-labeled polysomes and associated RNAs from target cell types. TRAP has the additional advantage of obtaining only translating mRNAs, which are a better proxy to the proteome than a standard mRNA preparation.