Early and accurate detection and isolation of the pathogens associated with endometritis in mares are crucial to initiating correct treatment in time, inhibiting bacterial resistance development, and optimizing fertility. This study aimed at the diagnosis of endometritis in mares through ultrasound examination followed by isolation, and identification of the causative microorganisms. In addition, the determination of the antibiotic susceptibility of the isolated microorganisms was carried out. Moreover, the in-vitro antibacterial efficiency of hydrogen peroxide (H2O2) against the isolated pathogens was assessed. Out of the examined 60 samples, 34 (56.7%), 30 (50.0%), 9 (15.0%), and 9 (15.0%) were confirmed positive for the presence of Pseudomonas aeruginosa, Escherichia coli, Serratia fonticola, and Klebsiella pneumoniae subsp. Ozaenae, respectively. Of the P. aeruginosa isolates, 76.5% showed intermediate resistance to gentamicin. E. coli showed 3.3%, 43.3%, 46.7%, 16.7%, 13.3%, 13.3%, and 26.7% resistance against cefazolin, gentamicin, amoxicillin/clavulanic acid, cefotaxime, ciprofloxacin, chloramphenicol, and tetracycline, respectively. Serratia fonticola isolates showed 100.0% resistance to each of gentamicin and amoxicillin/clavulanic acid while it showed 44.4% resistance to cefazolin and 66.7% resistance to each of cefotaxime, ciprofloxacin, chloramphenicol, and tetracycline. Klebsiella pneumoniae subsp. ozaenae showed 22.2% and 44.4% intermediate resistance against amoxicillin/clavulanic acid and cefazolin, respectively. Concerning the H2O2, the mean inhibition zones' diameters (mm) were 54.67, 48.33, 46.33, and 49.67 against Pseudomonas aeruginosa, Escherichia coli, Serratia fonticola, and Klebsiella pneumoniae subsp. ozaenae, respectively. This study demonstrated the very high in-vitro bacteriostatic and bactericidal efficiency of H2O2 at a concentration of 0.047% against bacteria-associated endometritis.