An analytical method to quantify dityrosine (DiTyr) in milk powder samples is presented. The assay is based on isotope dilution liquid chromatography coupled to electrospray ionisation tandem mass spectrometry (LC-ESIMS/MS). The sample preparation entails acid hydrolysis of milk proteins followed by a solid phase-extraction (SPE) step. Neither artifactual formation nor degradation of DiTyr were observed during the proteolysis step. Mass spectral detection was performed in the positive ion mode by recording five transition reactions for DiTyr, in order to unambiguously confirm the presence of DiTyr by correct ion ratios. Under the analytical conditions used, the limit of detection (LOD) and limit of quantification (LOQ) for DiTyr were estimated at ca. 2 and 6 micromol DiTyr per mol of Tyr (using ca. 500 microg of milk proteins), with a mean recovery of ca. 90%. Quantification was conduced in eight different commercial milk powder samples, and the level of DiTyr ranged from below the LOQ up to 393.0 +/- 9.1 micromol DiTyr per mol of Tyr.