Electrospray ionization and matrix-assisted laser desorption/ionization, two new mass spectroscopy methods for the accurate measurement of molecular masses of individual peptide and protein molecules, are finding great utility for the solution of problems in biological research. Thus, mass spectrometry is being used for the rapid identification and detailed characterization of proteins, the determination of modifications in proteins, and the assessment of the integrity and purity of (native, recombinant, or synthetic) protein preparations. Recent data indicate that mass spectrometry can contribute significantly to the study of protein interactions and even to the investigation of aspects of protein folding and conformation.