Thirty-seven mares, barren for at least one year, were diagnosed to have endometrial inflammation by cytological examination and/or bacterial culture. Sixteen of these mares (the control group) were treated with prostaglandin and reevaluated cytologically within 10 days of behavioral estrus. Fifteen out of sixteen (15/16) (93.8%) remained positive for endometrial inflammation. Twenty-one mares (immunologically treated group) were treated with Propionibacterium acnes by injecting 1 ml per each 113.3 kilograms body weight intravenously on days 1, 3, and 7 following the diagnosis of endometrial inflammation. They also received prostaglandin. Twelve of the immunologically treated mares were examined cytologically as were the control mares. One of twelve examined (1/12) (08.3%) was positive cytologically for endometrial inflammation when examined within 10 days after behavioral estrus. The other nine mares in the treated group were bred on the first behavioral estrus following treatment. Nine of nine (9/9) were determined to be pregnant to 60 days gestation. The other treated mares were bred following the cytological evaluation. Eighteen of the twenty-one treated mares (18/21) (85 7%) were confirmed pregnant to 60 days gestation.