The first stable isotope dilution assay for the determination of enniatins A, A1, B, and B1 and beauvericin was developed. The (15)N(3)-labeled enniatins and beauvericin were biosynthesized by feeding two Fusarium strains Na(15)NO(3) and subsequently isolated from the fungal culture. The chemical structures of the biosynthesized products were characterized by LC-MS/MS and (1)H NMR. Standard solutions of (15)N(3)-labeled beauvericin, enniatin A, and enniatin A1 were accurately quantitated by quantitative NMR. On the basis of the use of the labeled products as internal standards, stable isotope dilution assays were developed and applied to various food samples using LC-MS/MS. The sample extracts were directly injected without any tedious cleanup procedures. The limits of detection were 3.9, 2.6, 3.7, 1.9, and 4.4 μg/kg for enniatins A, A1, B, and B1 and beauvericin, respectively. Limits of quantitation were 11.5 (enniatin A), 7.6 (enniatin A1), 10.9 (enniatin B), 5.8 (enniatin B1), and 13.1 μg/kg (beauvericin). Recoveries were within the range between 90 and 120%, and good intraday and interday precisions with coefficients of variation between 1.35 and 8.61% were obtained. Thus, the stable isotope dilution assay presented here is similarly sensitive and precise but more accurate than assays reported before. Analyses of cereals and cereal products revealed frequent contaminations of barley, wheat, rye, and oats with enniatins B and B1, whereas beauvericin was not quantifiable.