The present study examines the possible interference of ceftibuten on intestinal microflora. In 60 germ-free mice, an intestinal microflora similar to the human one was implanted. The mice were then treated with a single dose and twice the therapeutic dose of ceftibuten. In another experiment, ten human volunteers were given ceftibuten in doses equivalent to 400 mg/day for 7 days in order to evaluate the ceftibuten faecal concentration, the possible interference on microflora and the quantity of indacan (as a measure of the bacterial metabolism in the gut) excreted in one day. The results obtained in vivo and in vitro have shown that ceftibuten in therapeutic doses does not alter the equilibrium of normal intestinal microflora.