Weanling pigs in groups of 12 were infected orally with Salmonella choleraesuis and were treated intramuscularly with doses of cephamycin C ranging from 12.5 to 337.5 mg twice daily for 10 days beginning 1 day postinoculation. Pigs in two other infected groups either received 300 mg of tetracycline orally on a similar schedule or served as nonmedicated controls. Optimal responses to cephamycin C were achieved at a twice daily dose of 112.5 mg. With this regimen, the febrile response was significantly reduced on day 2 and eliminated by day 5 postinfection, and the shedding of Salmonella spp. in feces was eliminated by day 5 postinfection; essentially, no lesions were found in the gastrointestinal tract at necropsy (day 26 postinfection). There was no mortality among recipients of the 112.5-mg dose; diarrhea was present on only 2% of the observation days. In contrast, 83% of the infected, nonmedicated pigs and 25% of the tetracycline-medicated pigs died, and diarrhea was present in these groups on 63 and 54% of the observation days, respectively. The striking benefits of cephamycin C treatment was achieved without adverse reactions. The weight gain and feed efficiency of the infected pigs treated with the 112.5-mg dose of cephamycin C and the noninfected, nonmedicated control pigs were equivalent.