An approach to drug discovery. Sites of interaction of several small molecules with a macromolecule, e.g. aminoglycoside antibiotics with rRNA, are simultaneously probed by MS: the addition to each small molecule of a tag of characteristic mass, which does not affect its binding to the macromolecule, directs each complex to a different region of the mass spectrum. Analysis of fragments by tandem MS reveals the specific region of the macromolecule to which the small molecules bind.