Endometritis is a common, costly, and frustrating disorder in horses that results in subfertility and poor reproductive performance in broodmares. Current therapies in the treatment of endometritis are generally supportive; uterine irrigation with isotonic fluids to remove inflammatory debris; anti-inflammatory therapies; and antibiotic infusions to prevent and treat bacterial growth and infection. Often, broodmares fail to respond to these treatments, and antimicrobial resistance is a common problem, so alternative treatment options are a popular area of research. There are few intrauterine therapies available that directly target the inflammatory response of the endometrium. One burgeoning therapy of interest is Mesenchymal Stromal Cell Conditioned Media (MSC-CM). Equine MSC-CM contains a milieu of bioactive factors that have been shown to promote tissue repair by way of paracrine signaling and immune modulation, antimicrobial peptide formation, and has also been demonstrated to prevent and destroy biofilm formation by way of targeting bacterial extracellular matrix (ECM). Our broad objective is to evaluate changes in the endometrial inflammatory response after treatment with MSC-CM. Initially, 6 estrus mares will be biopsied and endometrial cell culture initiated for in vitro study. Cells will be hormonally supplemented with estradiol or progesterone, and will then be treated with MSC-CM or DMSO for control. Supernatant from the cell culture before and after treatment will be analyzed for cytokine expression using a validated cytokine Multiplex assay quantifying inflammatory biomarkers, IFN-α, IFN-γ, IL4, IL17, IL10, IL1-β, CCL2, CCL3, CCL5, CCL11, TNF-α, and sCD14. Cells will then be lysed and RNA extracted for qPCR to evaluate for gene expression of IL-6, IL-8, PTGS2, PTGES, PTGIS, TNFα and TGFB, cytokines involved in the inflammatory pathway in equine endometritis. In the second part of the study, we will evaluate MSC-CM as a uterine infusion. Twelve endometritis susceptible mares in estrus will be used in the study; mares will randomly be enrolled in either the treatment or the control group. Mares will be sampled immediately prior to and 24 hours after infusion with either MSC-CM or plain saline. Outcome will be measured via ultrasound parameters, cytology, culture, and Multiplex cytokine analysis. Aim 1 will be to evaluate changes in gene expression and the cytokine profile of cells treated with MSC-CM in vitro and Aim 2 will be to evaluate changes in inflammatory markers in the uterus as well as differences in the clinical response to treatment. Our hypotheses are that treatment with MSC-CM will alter inflammation markers and gene expression in vitro, and improve outcome after intrauterine treatment by way of modulating the inflammatory response of the equine endometrium. This research is important because it will test the immunomodulatory effects and clinical efficacy of an antibiotic-sparing, naturally occurring equine biologic product in the treatment of equine endometritis. The results of this study may shed light on immunomodulatory interventions in equine endometritis and may help to guide further endeavors to pursue techniques in regenerative medicine in the treatment of equine endometritis, and likely reduce the overuse of intrauterine antimicrobial therapy in equine reproduction.