There is a growing interest in intestinal flora and human health and disease. The intestines of humans contain 100 trillion viable bacteria. These live bacteria, which make up 30% of the faecal mass, are known as the intestinal flora. There are two kinds of bacteria in the intestinal flora, beneficial and harmful. In healthy subjects, they are well balanced and beneficial bacteria dominate. Beneficial bacteria play useful roles in the aspects of nutrition and prevention of disease. They produce essential nutrients such as vitamins and organic acids, which are absorbed from the intestines and utilised by the gut epithelium and by vital organs such as the liver. Organic acids also suppress the growth of pathogens in the intestines.Other intestinal bacteria produce substances that are harmful to the host, such as putrefactive products, toxins and carcinogenic substances. When harmful bacteria dominate in the intestines, essential nutrients are not produced and the level of harmful substances rises. These substances may not have an immediate detrimental effect on the host but they are thought to be contributing factors to ageing, promoting cancer, liver and kidney disease, hypertension and arteriosclerosis, and reduced immunity. Little is known regarding which intestinal bacteria are responsible for these effects. A number of factors can change the balance of intestinal flora in favour of harmful bacteria. These include peristalsis disorders, surgical operations of stomach or small intestine, liver or kidney diseases, pernicious anaemia, cancer, radiation or antibiotic therapies, immune disorders, emotional stress, poor diet and ageing. However, more importantly, the normal balance of intestinal flora may be maintained, or restored to a normal from an unbalanced state, by oral bacterio-therapy or by a well balanced diet. Oral bacterio-therapy using intestinal strains of lactic acid bacteria, such as lactobacillus and bifidobacteria, can restore normal intestinal balance and produce beneficial effects. Benefits include suppression of intestinal putrification so as to reduce constipation and other geriatric diseases; prevention and treatment of diarrhoea including antibiotic-associated diarrhoea; stimulation of the immune system; and increased resistance to infection.