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The activities of mecillinam and ampicillin, alone and in combination, were evaluated in mice infected with the LT-2 strain of Salmonella typhimurium. The minimal inhibitory concentrations of mecillinam and ampicillin for this strain were, respectively, 6.2 and 0.4 ,tg/ml of culture medium. In vitro synergy was demonstrated. CF-1 mice inoculated intraperitoneally with 104 colony-forming units of the LT-2 strain were used in the therapeutic assessments. Treatment of subgroups with graded doses of the respective penicillins or their combination was initiated 24 h after inoculation and repeated at 6-h intervals for 5 consecutive days. Animals were observed during 21 days for mortality or sacrificed for quantitative cultures of spleen homogenates at the end of the treatment. Ampicillin in doses of -0.03 mg and mecillinam in doses of -0.5 mg reduced mortality rates from 77% in the saline-treated controls to a range of 0 to 47% (P< 0.05). The same doses of antibiotics also extended the median times to death and lowered significantly the means of splenic bacterial counts. When both drugs were combined in doses that were partially effective or subinhibitory alone, no synergistic effects were observed. These results showed that mecillinam and ampicillin given alone were effective in treating S. typhimurium infection but that combinations of the two drugs were not synergistic in controlling the course of infections.