It has been difficult to document the postulated sequence of events that begins with the selection of drug-resistant organisms in animals fed subtherapeutic amounts of antimicrobials and ends with clinically important infections in human beings. In early 1983 we identified 18 persons in four Midwestern states who were infected with Salmonella newport that was resistant to ampicillin, carbenicillin, and tetracycline and characterized by a 38-kilobase R plasmid. Twelve of these patients had been taking penicillin derivatives for medical problems other than diarrhea in the 24 to 48 hours before the onset of salmonellosis. Eleven patients were hospitalized for salmonellosis for an average of eight days, and one had a fatal nosocomial infection. We compared plasmid profiles of all human (six-state area) and animal (United States) S. newport isolates over an 18-month period and examined selected records of meat distribution. The results indicated that the patients had been infected before they took antimicrobials, by eating hamburger originating from South Dakota beef cattle fed subtherapeutic chlortetracy-cline for growth promotion. This study demonstrates that antimicrobial-resistant organisms of animal origin cause serious human illness, and emphasizes the need for more prudent use of antimicrobials in both human beings and animals. (N Engl J Med 1984; 311:617–22.). VARIOUS gastrointestinal illnesses — from mild, self-limited diarrhea to pseudomembranous co-litis — are recognized complications of treatment with antimicrobials.1 Less appreciated, however, is the clinical expression of previously asymptomatic infections with antimicrobial-resistant enteric bacteria after the use of antimicrobials. Only a single case of severe illness due to antimicrobial-resistant salmonella beginning after antimicrobial therapy has been previously reported.2 Multiple drug-resistant isolates have accounted for a steadily increasing percentage of human salmonella infections3 and now represent approximately 20 to 25 per cent of identified cases.4,5 The source of these resistant enteric pathogens in persons is controversial,6 7 8 but many believe that subtherapeutic.