Formal, Samuel B. (Walter Reed Army Institute, Washington, D.C.), G. D. Abrams, H. Schneider, and H. Sprinz. Experimental Shigella infections. VI. Role of the small intestine in an experimental infection in guinea pigs. J. Bacteriol. 85:119-125. 1963.-Shigella infection under the conditions of our experiment significantly involves the small intestine, producing lesions similar to those seen in human acute small intestinal dysentery. Previous work established that, for a rapidly fatal infection to occur, animals had to be pretreated by starvation or by administration of carbon tetrachloride, and, after oral challenge with S. flexneri, had to receive opium. Data are presented indicating that, in the guinea pig, the motility of the small intestine is a major defense mechanism in clearing bacteria from the gut. It is concluded that the two modes of pretreatment potentiate the ability of opium to decrease this motility, and hence increase susceptibility to enteric infection.