Methods and reagents for obtaining simplified mixtures of peptides from a sample containing a number of peptides are disclosed. The simplified sample can be easier to analyze than the original peptide sample yet it is representative of all or nearly all of the proteins present in the mixed protein sample from which the original and more complex peptide sample was derived. The methods entail the use of tagging moieties that include an amino-acid-specific reactive group (R). The tagging moieties "tag" peptides or proteins at specific amino acids (e.g., by reacting with an amino acid to form a covalent bond), ultimately allowing the isolation of peptides that contain those specific amino acids. Other methods entail the used of a reactive moiety (R.sub.P) that comprises a reagent that selectively interacts with selected proteins, either covalently or noncovalently. For example, R.sub.P can be a natural ligand for a receptor that is to be tagged or a protein that interacts with a second protein that is to be tagged. It can be an enzymatic substrate or other element of molecular recognition such as an antibody, ATP, GTP, NAD, NADP, NADH, NADPH, ubiquitin, or structural analogs thereof.