Although total bacterial counts were not reduced by bacitracin, E. coli counts were low in most parts of the intestine. This result is not easy to explain, since the Enterobacteriaceae including E. coli, are resistant to bacitracin (Weinstein, 1975). A possible explanation is that this is an effect mediated by the action of bacitracin on intestinal bacteria other than E. coil, with the net result that other bacteria, that compete with E. coli, are increased in number at the expense of E. coil counts.