Non-metallic inclusions (NMIs) are inevitably formed during steel production, affecting product quality. To investigate the origin of NMIs, active tracing methods adding specific compounds are state-of-the-art, which, however, influence the formation and properties of NMIs during steel production. Therefore, a novel approach has been developed and applied to overcome these drawbacks. The approach uses isotopic tracing, where the isotopic pattern of one element of a potential NMI source is modified by adding small quantities of stable isotope tracers of that element. In this study, slag enriched with 26Mg was prepared and applied in two high-resistance furnace experiments using either MgO or Al2O3 crucibles on a laboratory scale. The amount of 26Mg added was 0.6 % and 0.03 % of the total slag and steel mass, respectively, which is 10-times less compared to state-of-the-art tracing methods. Analysis of 26Mg/24Mg isotope ratios in potential Mg sources and several multiphase NMIs was conducted by laser ablation inductively coupled plasma mass spectrometry, and the amount of spike was computed via isotope pattern deconvolution. This revealed interactions between slag, steel, and refractories that were unambiguously traced back to the 26Mg-enriched slag, confirming the method's effectiveness for assessing NMI origins and modifications.