Amoxicillin-d4 is intended for use as an internal standard for the quantification of amoxicillin by GC- or LC-MS. Amoxicillin is an orally bioavailable, semisynthetic β-lactam antibiotic. It inhibits the growth of 30 isolates of P. mirabilis and 89% of 30 E. coli strains when used at concentrations greater than or equal to 5 and 10 µg/ml, respectively, but resistance develops in strains of Klebsiella, Enterobacter, and indole-positive Proteus species. Amoxicillin is susceptible to bacterial β-lactamases but is active against β-lactamase-producing bacteria when used in combination with β-lactamase antibiotics such as clavulanic acid with MIC values of greater than 4,096 and 16 µg/ml without or with clavulanic acid, respectively, against 46 clinical isolates of β-lactamase-producing E. coli. Formulations containing amoxicillin have been used in the treatment of a variety of bacterial infections.