Virginiamycin complex is defined as a mixture of 75% ostreogrycin A (virginiamycin M1) and 25% virginiamycin S1, together with the less abundant S analogues. As the two major components have quite different solubilities, these proportions are not readily achieved or used. BioAustralis has isolated and re-combined the individual components to provide the defined components of virginiamycin complex. The composition of the complex is important as Virginiamycin S1 acts a synergist, binding to the conformational change of the peptidyl transferase centre of the 50S ribosome induced by ostreogrycin A.