1. Preliminary treatment by mouth with a large dose (50 mg) of streptomycin increased the susceptibility of mice to infection following oral inoculation with a streptomycin-resistant strain of Salmonella enteritidis. In mice treated with streptomycin 24 hours before inoculation, <3 Salmonella sufficed to initiate infection in 50% as compared with approximately 10⁵ in untreated controls. 2. This effect of streptomycin decreased as the interval between treatment and inoculation was lengthened but was still detectable on the 5 th day. Smaller doses of streptomycin (5-10 mg) resulted in smaller increases in susceptibility. 1 mg was ineffective. 3. Representative numbers of mice killed for culture showed the spleen to be infected in 90% and heart's blood in 72% of those with positive fecal cultures at the time of autopsy. 4. It is believed that this increase in susceptibility following streptomycin treatment resulted from a disturbance of the normal intestinal microflora caused by the antibacterial action of the drug. 5. It is suggested that this method may be applicable to the experimental study of other enteric infections.