Acetylspiramycin was superior to other antibiotics to reduce total bacterial numbers in all parts of the gastrointestinal tract. A reduction in total bacteria counts could account for the improvement in animal performance since it is well established that microorganisms compete with the host for available nutrients (Wallace, 1970). In addition, it has been suggested that a reduction in intestinal bacteria may result in a thinner intestinal wall and thus facilitate nutrient absorption (Wallace, 1970).