The carbon stable isotope ratio (delta(13)C) and the alpha-linolenic acid (C18:3omega3) content of milk fat were analyzed to examine their applicability as general markers for the authentication of organic retail milk in Germany. To record the variable effect of feeding, including the seasonal influence on milk composition, three conventionally and three organically produced brands of retail milk were collected biweekly during a period of 18 months. Altogether 286 milk samples were analyzed. Threshold values for the identification of German organic milk were established and allowed to delimit almost all conventional samples. Organic retail milk was always above a minimum C18:3omega3 content of 0.50% and below a maximum delta(13)C of -26.5 per thousand. The universal and strongly negative correlation (r = -0.93) between C18:3omega3 and delta(13)C impedes the intentional manipulation of conventional milk. Conventional milk can naturally exceed the C18:3omega3 limit under atypical and rare conditions, but differentiation from organic milk can be improved by time-resolved comparison of data. In contrast with the general opinion, organic milk did not generally contain more c9,t11-C18:2 (CLA) than conventional milk. The proposed limits may deviate with dairy products containing milk from foreign countries.