For decades, infectious endometritis has been a major cause of infertility in mares and an inflammatory response secondary to uterine infection appears to be a major contributor to a suboptimal uterine environment. The aim of the present study was to characterize the endometrial gene expression of the glycoprotein lactoferrin in brood mares and to evaluate if the expression was associated with infectious endometritis. Endometrial biopsies were obtained from broodmares at a Danish AI-center during the 2014 breeding season by the use of a guarded approach as described by Nielsen (Nielsen, 2005). Mares with clinical signs of endometritis and/or a history of previous un-successful breeding were selected for the study. Two biopsies were obtained from each mare. One biopsy was used for bacterial culture and cytology and one biopsy was used for RNA extraction. Relative gene-expression analyses were performed by quantitative reverse transcriptase PCR (qRT-PCR) using validated primers and SYBR green detection. Infectious endometritis was diagnosed in 49% of the mares (29/59) with S. zooepidemicus isolated most frequently (45%). Data was analysed using the non-parametric Kruskal-Wallis test. Expression of lactoferrin was significantly increased in mares with infectious endometritis compared to mares with no growth of uterine pathogens. Lactoferrin expression was also associated with positive cytology (>0.5% PMNs) (Nielsen, 2005). The results indicate that lactoferrin plays a role in the innate immune response and emphasizes its ability to function as a bacteristatic and bactericidal agent.