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In vitro antimicrobial and antioxidant activities of Amaranthus retroflexus leaves and inflorescence alcoholic (ethanol 70%) extracts of various concentrations ranging from 0.78 to 400 μL/ml were analyzed on different clinical and reference bacterial strains (Staphylococcus aureus, Bacillus subtills, Enterococcus faecalis, Klebsiella pneumoniae, Escherichia coli, Pseudomonas aeruginosa, Acinetobacter baumannii) and fungal strains (Candida albicans, C. famata, C. utilis, Saccharomyces cerevisiae) using agar disk diffusion method and broth dilution method (MIC determination) for antimicrobial activity and TEAC (Trolox capacity) assay for the evaluation of the antioxidant activity. The adapted diffusion method was used to test the antimicrobial effect of the extracts separately and in combination with a range of standard antibiotics, in order to evidence any synergic effects of A. retroflexus extracts on the antibiotics activity. The extracts showed the highest inhibitory effect against K. pneumoniae and B. subtilis with no activity against S. aureus among the bacterial strains, while in case of the fungal strains the most intensive effect was exhibited against C. famata by both extracts. The A. retroflexus leaves extract improved the ciprofloxacin and ticarcillin-clavulanic acid activity towards the P. aeruginosa clinical strain. The inflorescences extract significantly increased chloramphenicol activity on B. subtilis strain. The antioxidant activity assay showed that the studied extracts exhibited the ability to neutralize the free radicals leading to the conclusion that the tested extracts bear compounds with a broad spectrum of antimicrobial and antioxidant activity that could represent a potential alternative for treating various infectious diseases.