The oral administration of cyclacillin, a semisynthetic aminoalicyclic penicillin, results in the elimination of the lactobacilli from the gastrointestinal tract of mice. Although cyclacillin has a broad spectrum of activity similar to that of ampicillin, it does not, like ampicillin, affect the other flora of the gastrointestinal tract. Cyclacillin, unlike ampicillin, is rapidly absorbed from the stomach and upper small intestine, so that only small amounts reach the large bowel. Study of the dynamics of the lactobacillus population in the gut shows that in the mouse the prime site of localization and multiplication of lactobacilli is the stomach. The lactobacilli are transient in the small and large intestine, arriving there only after having been shed from the stomach.