The antibacterial activity of NaFosfomicin and of two of its aminoacidic derivatives -Lisin-Fosfomycin and Arginin-Fosfomycin- has been determined "in vitro" as MICs and MBCs, on different groups of Bacteria. The results obtained were then statistically analyzed - according to the "t of Student" - in a comparative evaluation of the values between NaFosfomycin-Lisin-Fosfomycin (F-LF); NaFosfomycin-Arginin-Fosfomycin (F-AF); Lisin-Fosfomycin-Arginin-Fosfomycin (LF-AF). From these comparisons, the data found demonstrated no significant differences among the three compounds, for Proteus, Coliforms and Pseudomonas groups, while Salmonella and Staphylococcus Genera showed good differences (P less than 0.05) from the comparison F-AF.