During the past decade, there has been an increased interest in the microflora of the gut and its effects on the host organism. Several factors have contributed to this development. These include more sophisticated techniques for isolating and enumerating the anaerobic bacteria, the availability of germ-free and pathogenfree animals as experimental models, use of antibiotics with selective action, and better understanding of the metabolism of gut micro-organisms.Much of the preliminary work has been done in animal models, and recent work in man indicates that similar situations exist in the two systems. Through the use of experimental models, it has been shown that the gastro-intestinal flora (I) affects the growth and development of the animal, ( 2 ) influences the nutritional requirements, (3) affects the morphogenesis of the gastro-intestinal tract, (4) modifies, through metabolic activity, both endogenous and exogenous substances presented to the gastro-intestinal tract and ( 5 ) plays an important role in preventing other 'foreign' organisms from becoming established in the gut.One can now approach with greater confidence the symbiotic and parasitic aspects of the relationship between the gut flora and the host animal.