In contrast to a cell's virtually static genome, the proteome, the protein complement expressed by an organism, continually changes in response to external stimuli and internal processes. Global gene expression analysis at the mRNA level (i.e., transcriptome) has recently become feasible based on the serial analysis of gene expression and oligonucleotide micro-array assays. These techniques allow the activation states of thousands of genes to be polled simultaneously for a tissue or cell population. However, assays that measure mRNA abundances rather than the functional gene products (i.e., proteins) are uninformative with regard to protein modifications, and can poorly reflect protein abundances due to differences in stabilities, expression rates, etc., for both the mRNAs and proteins. The authors have developed an approach utilizing organisms cultured in stable-isotope labeled media (e.g., rare-isotope depleted and normal) to provide effective internal calibrants for all detected proteins, thus enabling precise proteome-wide measurement of changes in protein abundances resulting from cellular perturbations. The two (or more) isotopically distinctive cell populations are mixed prior to sample processing steps, eliminating all experimental variables associated with cell lysis, separation, and mass spectrometric analysis. Changes in relative protein abundances are thus precisely reflected by the ratio of two isotopically different and resolvable versions of each protein.