The aim of this study was to investigate the antibacterial activity and toxicity for sperm cells of the natural substances gallic acid, methyl gallate, ethyl gallate, propyl gallate, octyl gallate, thymol, carvacrol and eugenol. The antibacterial activity of these natural substances and selected combinations of them against bacterial strains isolated from boar ejaculates was determined using the microdilution and macrodilution method in Mueller-Hinton broth. The most effective natural substances against Gram-negative and Gram-positive bacteria included in our study were thymol and carvacrol with minimum inhibitory concentration (MIC) values in the range of 300-600 μg/ml. Gallic acid exhibited the best antibacterial activity against Pseudomonas aeruginosa strains (MIC values of 300-2400 μg/ml), whereas the ranges of MIC values against Escherichia coli, Enterococcus faecalis and Staphylococcus sp. strains were higher. Octyl gallate exhibited stronger antibacterial activity against staphylococci and enterococci (MIC values of 18.8-75 μg/ml) than against Escherichia coli and Pseudomonas aeruginosa strains with MIC values in the ranges of 300-600 μg/ml and 1200-2400 μg/ml, respectively. Thymol combined with carvacrol was the most effective combination against enterococci (MIC values of 75-300: 150 thymol: carvacrol) and Pseudomonas aeruginosa (MIC values of 75-300: 300 thymol: carvacrol), bacteria which are known to be frequently resistant to antimicrobials. Similar results were determined for the combination of carvacrol and eugenol against staphylococci and enterococci. The results of the combinations revealed more of an additive rather than a synergistic effect. Thymol and carvacrol were the most effective natural substances against the bacteria included in this study, with a low toxicity for sperm cells compared to other substances, suggesting their possible use for boar semen decontamination.