This paper examines the sources and nature of Locke’s medical thought. It is argued from a sampling of entries in Locke’s medical notebooks and his correspondence, that Locke was a chymical physician. His medical thought contains two interlocking strands: he was an adherent of mercurialist transmutational alchemy and Helmontian iatrochemistry. The major, though not the only, influence on these aspects of Locke’s thought was Robert Boyle.