Updated project metadata. Retained placenta (RP), a quite common disorder in dairy cows, shows a high negative impact on their health status and milk production. Therefore, the biomarkers indicating the risk for RP could yield benefits in dairy management. The study aimed to investigate the serum proteome as the source of the biomarkers for diagnosing the RP. Using the high-resolution liquid chromatography-tandem mass spectrometry, we compared the composition of serum proteome between 9 cows with the RP and 6 with the physiologic puerperium (PP). Among 651 identified proteins, seven had higher, and three showed lower abundance in RP than in the PP group. The differently abundant proteins participated in 15 pathways: six related to hemostasis, three involved in the lipoprotein metabolism, and the remaining ones associated with redox homeostasis, post-translational modification, scavenging, etc. Validation of the proteomic results, using ELISA and spectrophotometric assays, showed that the two identified proteins, haptoglobin, and lipopolysaccharide-binding protein, could represent reliable RP biomarkers. Our results confirmed that the analysis of serum proteome shed "the light" on the counterparty piece(s) of the RP pathogenesis "puzzle" and could be an efficient "tool" in mining for RP biomarkers.