The aim of this study was to investigate the effect of ampicillin on the ability of the human colon to ferment carbohydrate. The effect of ingesting a drink containing 20 g of lactulose on stool output and breath hydrogen production was measured in 13 normal volunteers before and during administration of ampicillin (2 g/day). Small bowel and whole gut transit times were also measured to exclude any direct effect of ampicillin on motor activity. Ingestion of lactulose did not increase stool weight or frequency under control conditions, but during administration of ampicillin, lactulose caused increases in stool weight (p less than 0.02) and frequency (p less than 0.01), in the percentage of unformed stools (p less than 0.001), and in the excretion of galactose and fructose in stool samples collected from 2 volunteers. Administration of ampicillin also significantly reduced the area under the breath hydrogen profile (p less than 0.03). Mouth-to-cecum transit of the lactulose drink was prolonged during ampicillin ingestion (p less than 0.01) but there was no significant change in the whole gut transit time. These results suggest that ampicillin impairs colonic fermentation of carbohydrate and a diet high in unabsorbable carbohydrate increases the risk of antibiotic-associated diarrhea.