Conventional Sprague-Dawley rats were fed by gastric tube for 5 days with either benzylpenicillin, ampicillin, doxycycline, or clindamycin. In contrast to the pretreatment period fibrinolytic activity and active and immunoreactive trypsin and immunoreactive elastase were present in fecal extracts after 4 days of antibiotic administration. This is consistent with findings in germfree rats and represents an alteration in the intestinal microflora. The germfree characteristics persisted until a suspension of cecal contents from normal rats was administered by enema on the 26th day. In fecal extracts from the clindamycin-treated rats considerable amounts of active and immunoreactive enzymes were, however, still found 10 days after the enema. A possible explanation is that clindamycin or a metabolite remains in the intestinal tract for a long time. The pathophysiological significance of this finding is unknown.