At the beginning of the last century, the Russian immuno- logist Elie Metchnikoff argued that life-long intake of yoghurt containing lactic acid-producing microorganisms could ex- plain the differences in length of life between ethnic groups. The idea was that the bacteria in the fermented products com- peted with microorganisms that are injurious to health. 1 Today it is known that the normal human microflora is import- ant as a barrier against colonization by exogenous pathogenic microorganisms and potentially pathogenic bacteria already present in small numbers in the microflora. 2 The normal microflora influence several biochemical, physiological and immunological features of the host, particularly the gastro- intestinal flora, which consists of the most dense and diverse collection of bacteria. 3 Disturbances in the normal microflora can be caused by several things, one being the administration of antimicrobial agents. 4 Probiotic microorganisms are thought to counteract disturbances and thereby reduce the risk of colonization by pathogenic bacteria. 5 Studies on strains of microorganisms used in probiotic dietary supplements have demonstrated that several strains produce antimicrobial sub- stances such as organic acids, bacteriocins and peptides. In vitro and animal studies have further shown inhibitory effects of probiotic bacteria to be mediated by their interference with the adhesion of gastrointestinal pathogens or with toxins pro- duced by the pathogenic microorganisms. Adjuvant-like effects on intestinal and systemic immunity have also been demonstrated for some strains. 6 However, it is important to remember that even closely related strains have hetero- geneous qualities. A number of clinical studies have been performed on the ability of probiotic strains to prevent or treat gastrointestinal infections. The most common strains belong to the two genera Lactobacillus and Bifidobacterium, but other microorgan- isms including Enterococcus, Streptococcus, Escherichia coli and Saccharomyces species have also been used. The results from well-performed, double-blind, placebo-controlled studies suggest that Lactobacillus strain GG and Saccharo- myces boulardii are the two most promising species in the prevention of diarrhoea. Lactobacillus GG has been effective in reducing the frequency and duration of rotavirus-induced diarrhoea in children. Recently, it was shown that in children admitted to hospital the risk of acquiring nosocomial diar- rhoea was reduced from 33% to 7% in the group receiving prophylactic therapy with Lactobacillus GG. 7 However, the