Feces from normal and antibiotic-treated persons were analyzed for the content of immunoreactive trypsin and elastase. In the control group the mean concentration of immunoreactive trypsin was 13 microgram per g feces as compared to 147 in the antibiotic-treated group. Elastase was demonstrable in only 3 of 30 samples in the control group but in 20 of 26 in the antibiotic-treated group. The decreased inactivation of pancreatic proteases must depend on an altered intestinal microflora. The results suggest that reestablishment of a normal enteric flora may take months after the short time oral administration of antibiotics.