In the laboratory investigation of suspected community acquired diarrhea, stool cultures may be ordered on patients receiving antibiotic therapy. Because many antibiotics cause profound changes in intestinal microbial flora, the value of these cultures is not known. To determine the effect of concurrent antibiotic therapy on results of routine stool cultures in the isolation of enteropathogens, a retrospective analysis of fecal cultures submitted during a 14-month period was performed. Of 930 specimens from 856 patients, there were 236 samples (25.4%)from 223 patients (26.1%) receiving antibiotics. Of these, 198 cultures (83.9%)from 186 patients (83.4%) receiving antibiotic therapy showed no growth on all media or growth of only gram-positive organisms. None of these specimens revealed enteropathogens. The results of this study demonstrate that there is little value in culturing of stools of patients receiving antibiotics. Significant cost savings can be realized by elimination of these cultures.