A process for the preparation of 14-O-[(N-(3-methyl-2-amino-butyryl-piperidinyl)sulfanyl) acetyl]mutilins of formula feasible for large-scale production of high purity products, and wherein the carbon atom at the piperidine ring attached to the sulphur atom is either in the (S)-configuration or in the (R)-configuration, and a new crystalline form of 14-O-[(N-3-methyl-2-(R)-amino-butyryl-piperidine-3(S)-yl)sulfanyl)acetyl]mutilin - hydrochloride.