Background. Triazoles and Schiff’s bases have a high biological activity. For the practical use of the derivatives, their low toxicity is important. The purpose of this work was to investigate the antibacterial and phytotoxic properties of Schiff’s bases of 5-phenyl-4-amino-3-mercapto-4H-1,2,4-triazole with donor substituents in the 4th position of heterosystem. Materials and methods. In the study of antibacterial activity of the derivatives, corrosion-active 4-day association cultures of ammonifying and sulfate-reducing bacteria were used as a test culture of microorganisms. Sensitivity of bacteria to derivatives was determined by diffusion method in agar using sterile paper disks according to the standard method. In the investigation of phytotoxic activity of the derivatives, Lepidium sativum of the “Ajour” cultivar was used as a test plant. Seed germination and biometric indices (length, weight of the aboveground part and roots) of 5-day sprouts were determined, the phytotoxic effect of the derivatives was calculated. Experimental data were processed using methods of mathematical statistics. Results. The introduction of substituents does not provide for an increase in antibacterial properties of the studied compounds in relation to some corrosion active ammonifying and sulfate-reducing bacteria. Low activity was observed regarding the association culture of ammonifying bacteria to the compound without substituents in the phenyl fragment and the compound with fluor as a substituent in the phenyl fragment at a concentration of 2.0%. Derivatives with the methoxyl substituent in the phenyl fragment and with the hydroxyl substituent in the phenyl fragment did not show any antibacterial activity against the association culture of ammonifying bacteria isolated from ferrosphere in meat-peptone broth. Antibacterial action against the association of sulfate-reducing bacteria Desulfovibrio orizae with organic acid-producing bacteria Anaerotignum propionicum for derivatives were not detected. Phytotoxic properties were observed for the compound with the hydroxyl substituent that influenced the processes of growth in the test plant. Conclusion. The introduction of electron-donor substituents into the basic structure did not provide for an increase in antibacterial properties against corrosive bacteria. Phytotoxic properties were observed for the compound with the hydroxyl substituent in the phenyl fragment, which influenced the L. sativum growth processes by inhibiting growth of the above-ground part and roots. Other compounds either did not show any action, or demonstrated a weak stimulating effect on the growth and development of the test plant.

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