The cyto-microbiological examination of bovine quarter milk samples is a widely used and proven method in Germany. The method is used to analyze clinical and subclinical mastitis milk with regards to the causative mastitis pathogen and the somatic cell count. In order to give mastitis labs, the opportunity to carry out internal quality control measures, the Udder Health Group of the German Veterinary Association has carried out a comparative ring trial for the 3rd time in 2019. Eight German mastitis labs took part in the study. The samples were submitted to the laboratory processes unrecognizable. The results showed good agreement regarding the cyto-microbiological findings and the results of the sensitivity testings between the participating labs. The outcome of the ring trial shows that the attending mastitis laboratories provide reliable results due to the high degree of standardization of their test procedures.