In recent years, the introduction of improved anaerobic culture technique and media, and the utilization of gnotobiotic animal technique have greatly promoted the studies of microecological research of intestine. Intestinal flora participates in normal physiological processes and also contributes to complex pathological states. The flora is capable of metabolizing a variety of substances such as foods, drugs or endogenous compounds, thus influencing drug efficacy and toxicity, carcinogenesis, and aging. However, little is known about which bacterial species implicate in these phenomena, and about the mechanism of these microbe-host interactions. A solution of these problems will only be achieved by the cooperation of workers of different research areas such as microbiology, physiology, biochemistry, pathology, pharmacology, toxicology and clinical medicine.