Tiamulin is a semi–synthetic analogue of pleuromutilin in which the hydroxyacetyl side chain is replaced with a larger diethylaminoethylthioacetyl moiety, providing greater hydrophobicity. The hemi-fumarate provides a stable salt with improved water solubility. Tiamulin is a potent and highly selective antibiotic active against a range of Gram positive bacteria, with no cross resistance to existing antibiotic classes due to its unique mode of action of inhibiting protein synthesis by binding to domain V of 23S rRNA.