Determination of sodium fluoroacetate in infant formula by liquid chromatography-tandem mass spectrometry (LC-MS/MS)is discussed. Sodium fluoroacetate is also a naturally occurring poison found in at least 40 plants native in Australia, South and West Africa, and Brazil. Milk powder is first reconstituted in water. Liquid sample is used as such. Acetonitrile is added to precipitate proteins. After centrifugation, the supernatant is washed with hexane and then acidified with concentrated sulfuric acid. QuEChERS salts are added for phase separation and the mixture is centrifuged. The resulting supernatant is evaporated to 0.5 mL remaining volume and centrifuged before LC-MS/MS analysis in selected reaction monitoring (SRM) by electrospray ionization (ESI) in negative mode.