Analysis of the stable isotope ratio of carbon (delta (13)C) and alpha-linolenic acid (C18:3omega3) content in milk fat is a useful indicator of organic milk production. Referring to corresponding measurements, further analyses of stable isotope ratios were performed in 120 samples of conventionally and organically produced whole milk collected from German retailers during a period of 18 months. Conventional milk predominantly exhibited higher delta (15)N values than organic milk, the latter of which never exceeded a maximum delta (15)N threshold value of 5.50 per thousand. Measurements of delta (34)S did not differ significantly between organic and conventional milk. Because delta (13)C, in general, is related to maize consumption, delta (13)C in milk protein and delta (13)C in milk fat were equally suited for authentication of organic milk. Thus, a high correlation (r = 0.99) was established between delta (13)C in milk protein and lipids. Although occurring on different levels in organic and conventional milk, the relatively constant fractionation of carbon isotopes between protein and fat will allow for the advanced detection of adulteration in processed milk products, such as fraudulent combinations of organic milk fat and conventional skim milk. In addition to the strong correlation between C18:3omega3 and delta (13)C(protein) (r = -0.91), a mutual dependence was identified between both delta (13)C(protein) and delta (15)N (r = 0.66) and C18:3omega3 and delta (15)N (r = -0.61). Thus, multi-variable analyses are useful to increase robustness and reduce the number of exceptions in organic milk authentication. Future work involving multivariate statistical analysis can possibly further improve milk authentication in various respects including differentiating between brands of retail milk.