Florida citrus depends on a breakthrough in the fight against citrus greening disease. Of the antibiotics used to treat this disease, penicillin G has been one of the most effective. Because orange fruit grown in the state of Florida are mainly used to produce orange juice, we have validated an ultra-HPLC tandem MS method to screen for penicillin G and its metabolites (penillic and penilloic acids) at the chemical residue level after treatment. In this method, three spike levels (0.25, 1, and 20 ng/g) were tested in triplicate. Absolute recoveries for penillic and penilloic acids were 60-75% depending on the matrix used, whereas corrected recoveries of penicillin G using an isotopically labeled internal standard were ~100%. Two product ion transitions per analyte were required for identification, which contributes to a high degree of selectivity.