The thesis deals with biological effects of kefir, which include modification of gut microbiota, antimicrobial activity, mediating inflammation and allergy response, antioxidative activity, inhibiting growth and proliferation of cancer cells, blood pressure reduction, modulation of lipids, glucose and insulin in bloodstream, impact on fat metabolism in liver and adipose tissue and reducing symptoms of lactose intolerance. Kefir is generally a good source of vitamins and some minerals, it also has beneficient fatty acid profile and contains a lot of bioactive peptides. Microorganisms of kefir are also very important because of their probiotic properties and their metabolic activity that continues in the gut after consumation. Different kefir components can affect different cell mechanisms with binding to specific cell surface receptors. They can also be transported into the cell and react with different molecules resulting in changes on proteins and even genetic material of the cells. Kefir consumation also changes the activity of epithelials cells through altering gut microbiome and gut microenvironment. Some components of kefir can also bind to toxic molecules and organisms in gut and inhibit their activity and prevent injuries.