Antibacterial activities of fosfomycin were investigated both in vitro and in vivo for the purpose of comparative evaluation on its fundamental properties with other antimicrobial agents. The MIC was determined with nutrient agar (Dif-co) inoculated with one loopful of 1,000-fold dilution (about 106 cells/ml) of bacterial suspension cultured overnight in nutrient broth. This substance showed antibacterial activity to most gram-positive and gram-negative bacteria, being strongest to Enterobacteriaceae with a peak of the MIC at 1.56 μg/ml in Salmonella. It was also active against P. aeruginosa with a peak of the MIC at 6.25 μg/ml in its sensitivity distribution. Intravenous and subcutaneous fosfomycin Na salt and oral fosfomycin Ca salt were given to 5-week-old ddN strain male mice challenged with clinical isolates of P. aeruginosa, E. coli and P. mirabilis. Therapeutic effect was observed in all these test organisms. In P. aeruginosa, it was more effective than car-benicilli