A natural preservative composition obtained from plant materials provides antimicrobial activity for various compositions such as a food composition. The antimicrobial agent is effective in inhibiting the growth of gram-positive Staphylococcus aureus, gram-negative Escherichia coli, Salmonella typhimurium, Klebsiella pneumoniae, and Pseudomonas aeruginosa, acid-fast bacterium Mycobacterium smegmatis, the yeast Candida albicans. The antimicrobial agent has a MIC of 3.0 .mu.l/ml capable of inhibiting these organisms. The antimicrobial agent includes mixtures of Origanum vulgare L., Thymus vulgaris L., Cinnamomum zeylanicum Nees, Rosmarinum officinalis L., Lavandula officinalis L., Hydrastis canadensis L. and olive leaf extract. The antimicrobial agent is an effective natural alternative to commonly used synthetic ingredients for product preservation.