338 Abbreviations Introduction R. Helal and M. F. Melzig: Concentration (conc.); human colon adenocarcinoma cell line (HT-29); lysozyme (lys.); phosphate buffered saline (PBS); saponin (sap.); standard deviation (SD); human acute monocytic leukemia cell line (THP-1). Human lysozyme (1,4-N-acetylmuramidase, E.C.3.2.1.17) is a ubiquitous low molecular– weight enzyme present in a wide range of biological fluids and tissues within animal and plant kingdoms [1]. Since the initial discovery of lysozyme in 1922 in tears and nasal secretions by Fleming [2], huge literature has accumulated on its structure, function, genetics, biosynthesis, regulation, enzyme activity and properties. Lysozyme is one of the most important factors of innate immunity, possessing antibacterial, antiviral, antitumor and immune modulatory activities [5]. Among major cell types responsible for the production of lysozyme are monocytes/macrophages and neutrophilic granulocytes as a part of their surveillance functions in the immune system [3]. In addition, this antimicrobial peptide tends to be found abundantly in animal tissues that are likely to come in contact with pathogens, such as airway epithelia and other epithelial tissues [4]. The purpose of the present study was to investigate whether in vitro exposure to various concentrations of selected saponins can affect the release and production of lysozyme activity in human monocytic cells THP-1, and in human epithelial cells HT-29. The selection of the investigated saponins was based on their proven antibacterial, antiviral, anti-inflammatory or immune-stimulating properties. Saponins are steroid or triterpenoid glycosides found in plants, lower marine animals and some bacteria [6, 7]. Saponins have been proved to have immunomodulating effects as well as antitumoral, antiviral and antimicrobial activity [8, 9]. They not only have stimulatory effects on the components of specific immunity, but also present some non-specific immune reactions such as inflammation [10, 11] and monocyte proliferation [12, 13]. Specific saponins have been reported to have the ability to modulate the cell mediated immune system as well as to enhance antibody production and have the advantage that only a low dose is needed for adjuvant activity [14]. The mechanisms of immune- stimulating action of saponins have not been clearly understood, but many explanations have been put forward. Some saponins have reportedly induced the production of cytokines such as interleukins and interferons that might mediate their immunostimulant effects [15, 16]. To investigate the possibility that lysozyme might serve as a useful marker of the immune modulatory properties of selected saponins and to examine the relative contribution of these natural substances to lysozyme production and release in monocytes and epithelial cells, lysozyme activity levels in culture fluids of these two cell types were measured in the present study using highly sensitive fluorescence-based assay [17]. Sci Pharm. 2011; 79: 337–349