An interlaboratory study was conducted to evaluate stable isotope dilution and LC tandem MS (MS/MS) for the determination of aflatoxins B1, B2, G1, G2, and M1 (AFB1, AFB2, AFG1, AFG2, and AFM1) in milk, milk-based infant formula (formula), and feed. Samples were first fortified with five 13C uniformly labeled aflatoxins {[13C]-internal standard (IS)} corresponding to the five native aflatoxins, which were subsequently extracted with acetonitrile-water (50 + 50, v/v), followed by centrifugation, filtration, and LC-MS/MS analysis. In addition to certified milk powder and animal feed, the three participating laboratories also analyzed milk, formula, and feed fortified with the five aflatoxins at concentrations ranging from 0.5 to 50 ng/g. The majority of recoveries ranged from 80 to 120%, with RSDs < 20%. Method LOQs were determined by the three laboratories using the three sample matrixes in replicates (n = 8), and the determined LOQs of AFB1, AFB2, AFG1, AFG2, and AFM1 ranged from 0.1 to 0.91, 0.24 to 0.64, 0.28 to 1.52, 0.19 to 3.80, and 0.12 to 0.45 ng/g, respectively. For detected aflatoxins in the certified materials, all measured concentrations were within ±25% of the certified values. Using [13C]-IS eliminated the need for matrix-matched calibration standards for quantitation, simplified sample preparation, and achieved simultaneous identification and quantitation of the aflatoxins in a simple LC-MS/MS procedure.