We report the simultaneous screening of highly polar, water-soluble, and less-volatile herbicides, including glyphosate, glufosinate, paraquat, and diquat, in serum using liquid chromatography-mass spectrometry. The herbicides were separated by solid-phase extraction using a Strata-XC cartridge. A heptafluorobutyric acid solution was chosen as the mobile phase for ion-pair liquid chromatography. Mass spectrometry was used for analysis and was optimized for operation in the positive mode for all analytes. The serum specimens were screened for the presence of the herbicides at the following concentrations: 5 ng/mL for glyphosate, 2 ng/mL for glufosinate, 1 ng/mL for diquat, and 5 ng/mL for paraquat. This is the first report on the simultaneous detection of these compounds.