Probiotics promote the establishment and development of a desirable intestinal microbial balance. There is a delicate balance between normal and pathogenic microorganisms. This balance can be upset by poor husbandry conditions, disease, or stressors (eg, transport). Bacteria that produce lactic acid can, in general, be beneficial to the animal; certain yeasts may also be beneficial. Their ability to increase growth and promote health are claimed to be due to one or more of the following factors: preventing colonization of the gut by pathogenic coliforms, altering GI absorption rate, and inhibiting bacterial growth and influencing the balance of bacteria in the gut. The probiotic feed additives consist of selected strains of lactobacilli and streptococci that alter the microbial species present in the GI system to the benefit of the treated animal. Unicellular yeasts are also used. The production benefits are variable, and positive responses are more likely when a stressful management change may result in a change in balance of gut microflora. Thus, they are useful in some cases to minimize GI upsets or to help overcome stress due to weaning or transport. The unicellular yeast fungus may also have beneficial effects on rumen fermentation, thereby improving digestion and feed efficiency. The effect of probiotics in older animals may be reduced because of the well-established, balanced population of microflora that is less sensitive to minor detrimental husbandry challenges.