Using L929 cells, we quantitated the macroeconomics of protein synthesis and degradation and the microeconomics of producing MHC class I associated peptides from viral translation products. To maintain a content of 2.6 × 10⁹ proteins, each cell's 6 × 10⁶ ribosomes produce 4 × 10⁶ proteins min⁻¹. Each of the cell's 8 × 10⁵ proteasomes degrades 2.5 substrates min⁻¹, creating one MHC class I-peptide complex for each 500–3000 viral translation products degraded. The efficiency of complex formation is similar in dendritic cells and macrophages, which play a critical role in activating T cells in vivo. Proteasomes create antigenic peptides at different efficiencies from two distinct substrate pools: rapidly degraded newly synthesized proteins that clearly represent defective ribosomal products (DRiPs) and a less rapidly degraded pool in which DRiPs may also predominate.