Abstract Background Equine embryonic loss following the development of endometrial cups delays return to cyclicity due to the production of equine chorionic gonadotropin (eCG). Natural degradation of endometrial cups coincides with an influx of immune cells at 100–120 days of gestation, but therapeutic stimulation of reduced eCG production has been relatively unsuccessful. Recently, we observed an increase in pro‐inflammatory cytokine production following the use of the immunostimulant mycobacterium cell wall fraction (MCWF). Objectives To evaluate the efficacy of hysteroscopic‐guided injection of MCWF on the accelerated decline of eCG secretion. Study design In vivo experiment. Methods Mares were pharmacologically aborted at 40–45 days of gestation, and then divided into groups: MCWF‐treated (6 mg MCWF suspended in 20 mL LRS; n = 10) and Control (20 mL LRS; n = 6). Five days after abortion, hysteroscopic‐guided injection of endometrial cups was performed, with 1 mL of volume placed into each visible endometrial cup. This was repeated 7 days later. Trans‐rectal ultrasonography was performed to monitor ovarian activity, and serum was obtained to assess eCG and cytokine concentrations. Results Concentrations of eCG decreased in the MCWF‐treated group ( p < 0.01) with a significant suppression noted as early as 14 days after onset of treatment and remained suppressed for the duration of the study. This coincided with an increase in peripheral IFN‐γ ( p < 0.01) and IL‐1β ( p < 0.01) concentrations. Eight out of ten MCWF‐treated mares (80%) developed pre‐ovulatory follicles, in comparison to 2/6 controls (33%). A pre‐ovulatory follicle was noted 23 ± 4 days after onset of treatment. Main limitations No pregnancy data was obtained following treatment. Conclusions This is the first report of a treatment for the accelerated reduction of eCG following abortion. Stimulation of this process allowed mares to develop a pre‐ovulatory follicle within a month of MCWF treatment onset, granting repeat attempts at breeding within the confines of a single breeding season.