Normal microflora of the digestive tract plays an important role in maintaining competence of the immune system. Imbalance of the flora may lead to the development of either diseases related to overgrowth of its selected constituents (post-antibiotic diarrhoea, travellers' diarrhoea or infection by external pathogens--rotavirus diarrhoea) or diseases resulting from altered immunological response (atopy, inflammatory bowel disease, neoplasmas). It is believed that application of probiotics may restore proper composition and functions of the microflora and thus bring new perspectives into prevention and treatment of these illnesses. Prospective studies on mechanisms of the probiotic activities may enable their new medical applications.