Antibiotics growth promotants (AGP), orally active at sub-therapeutic levels, have provided a dietary means of efficiently producing pork, beef, and poultry meat. AGPs increase weight gain in pigs and broilers. The proposed mechanisms involve inhibition of subclinical infections, reduction of growth-depressing microbial metabolites, reduction in competing microbial use of nutrients, enhanced uptake and use of nutrients, and reduced protein turnover associated with immune stimulation. Ionophore antibiotics disrupt ions transport in the cell and are used as coccidiostats in poultry to avoid serious intestinal damage. A major impediment to the use of AGP, however, is the risk of creating antibiotic-resistant microorganisms.