A technique that combines ion mobility spectrometry (IMS) with reversed-phase liquid chromatography (LC), collision-induced dissociation (CID) and mass spectrometry (MS) has been developed. The approach is described as a high throughput means of analysing complex mixtures of peptides that arise from enzymatic digestion of protein mixtures. In this approach, peptides are separated by LC and, as they elute from the column, they are introduced into the gas phase and ionised by electrospray ionisation. The beam of ions is accumulated in an ion trap and then the concentrated ion packet is injected into a drift tube where the ions are separated again in the gas phase by IMS, a technique that differentiates ions based on their mobilities through a buffer gas. As ions exit the drift tube, they can be subjected to collisional activation to produce fragments prior to being introduced into a mass spectrometer for detection. The IMS separation can be carried out in only a few milliseconds and offers a number of advantages compared with LC-MS alone. An example of a single 21-minute LC-IMS-(CID)-MS analysis of the human plasma proteome reveals approximately 20,000 parent ions and approximately 600,000 fragment ions and evidence for 227 unique protein assignments.