Swiss white mice were given ampicillin, clindamycin, kanamycin, metronidazole, or streptomycin in drinking water for a period of 3 weeks. One week after the initiation of antibiotic administration, the treated mice and untreated control mice were challenged orally with approximately 108 viable, streptomycin-resistant (SR) Pseudomonas aeruginosa isolates. All five of the antibiotics decreased the resistance of the mice to intestinal colonization with SR P. aeruginosa, as reflected by an increased fecal carriage of the organism and an increase in population levels of SR P. aeruginosa in feces as compared with untreated controls. Metronidazole was least effective in this regard. The antibiotics lowered the dose of SR P. aeruginosa that resulted in implantation in 50% of the mice ID50 to various degrees. Administration of streptomycin, the most effective antibiotic, caused a 10,000-fold decrease in ID50 as compared with untreated controls. Oral inoculation of approximately 108 organisms of SR P. aeruginosa resulted in translocation of the organism to the * Corresponding author.