Quinupristin-dalfopristin mesylate complex is a 70:30 (w/w) complex of two semi-synthetic analogues marketed as Synercid. Dalfopristin is a semisynthetic analogue of virginiamycin M, while quinupristin is a semi-synthetic analogue of virginiamycin B. To optimise stability, the compounds are presented as the mesylate salts with 10% sodium mesylate excess to provide a buffered aqueous solution. The complex is more hydrophobic than the naturally-occurring virginiamycin complex, with a readily ionisable group for generating a salt for improved solubility. There is little published data on the synthesis and biological activity of individual components of the complex, however the combination is a highly effective antibiotic, including activity against resistant strains.