This standard specifies the liquid chromatography-tandem mass spectrometry method for the determination of warnemulin hydrochloride and tiamulin hydrochloride in feed. This standard is applicable to the determination of warnemulin hydrochloride and tiamulin hydrochloride in compound feeds, concentrated feeds, concentrate supplements, and additive premixed feeds. The detection limits of this standard for warnemulin hydrochloride and tiamulin are both 0.1 mg/kg, and the quantitation limits are both 0.5 mg/kg.