Objective: Especially in cases of clinical mastitis a rapid test for identification of responsible pathogens would be useful for choosing the appropriate treatment. The objective of the study was to determine whether two rapid tests are suitable to detect infections with coliforms and Staphylococcus aureus, respectively, in cows with acute mastitis under field conditions. Material and methods: The evaluated tests were "Petrifilm™ Rapid Coliform" and "Petrifilm™ Staph Express Count System". A total of n = 107 double, quarter foremilk samples from dairy cows with clinical signs of acute mastitis was obtained aseptically. All samples were examined microbiologically by means of both conventional methods and using both Petrifilm™ tests. Results: Overall sen sitivity for the Petrifilm™ tests was 32%, overall specificity 90%, and the positive predictive value 24%. In contrast, conventional methods yielded positive results in only 18% of cases, meaning that with the petrifilm kits, 3.5 times more infections caused by coliforms could be detected. However, not all coliforms cultivated conventionally were also identified by the Petrifilm™ tests. Conclusion and clinical relevance: These unsatisfactory results may be due to flaws in the conventional culturing or contamination of samples. Therefore, the results of this field trial do not lead to a conclusive evaluation of this diagnostic device under field conditions, although the Petrifilm\; test for coliforms may already be recommended to identify infections caused by gram-negative microorganisms. Once caution is taken at sampling, this kit may lead to a basic decision regarding therapy.