Ciprofloxacin was compared with erythromycin for the eradication of Campylobacter species that were chronically excreted in the stools of marmosets (Saguinus labiatus labiatus, Saguinus fuscicollis nigrifrons, and Saguinus fuscicollis illigeri). Stool cultures were negative within 48 h of the beginning of treatment with either agent. Within 10 days after the end of therapy, however, Campylobacter species were again isolated from the stools of six animals that had received erythromycin. During an 8-week follow-up period, no animal that had received ciprofloxacin relapsed. High levels of ciprofloxacin in the stool (mean, 49.2 micrograms/g) possibly contributed to the efficacy of this agent.