Objective: To evaluate the efficacy of hyperimmune serum (HS) in treatment of postpartum endometritis. Methods: In a field trial, cows with vaginal discharge, 25-35 d in milk were randomly assigned to three treatment groups. In group 1 (n=42), cows received an intrauterine treatment with 50 mL HS. HS was produced against Arcanobacterium pyogenes and Escherichia coli that had already been isolated from the Iranian dairy farms. In group 2 (n=39), cows were treated with one intrauterine infusion

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... 5 g oxytetracycline (OTC). In group 3 (n=65), cows affected with endometritis were treated with 0.5 mg cloprostenol (PG). Group 4 (n=89) included clinically healthy cows as control (HC) group without any treatment in groups HS, OTC and PG; all cows were re-examined 39-49 d in milk. Reults: Cure rate after treatment defined as the absence of vaginal discharge at the reexamination, was 64.3%, 61.5% and 72.3% in HS, OTC and PG groups, respectively (P>0.05). Cows categorized as E1 and E2 showed higher cure rate and reproductive performance measures than E3 cows in both treatment groups but their differences were not significant. Conception rate to all services for cows with endometritis (category E1, E2 and E3) was 52.9% in HS group, 57.1% in OTC and in PG 62.1% compared to 66.7% in HC (P>0.05). The difference in the conception rate between the HS and HC group was not significant. Conclusions: We suggest that HS could be the no antibiotic alternative treatment choice for postpartum endometritis in dairy cattle.