The clinical and bacteriological response of 38 treatments performed on 24 children (11 of them neonates) carrying out separate treatments with carbenicillin (2 treatments), gentamicin [4], fosfomycin [6], and associated treatments with gentamicin plus carbenicillin [6], fosfomycin plus gentamicin [18] and fosfomycin plus carbenicillin [2] are considered. The clinical cure was obtained in 21 children (87.5%). The most effective treatment was fosfomycin plus gentamicin; both antibiotics showed synergism in vitro on isolated Serratia strains. A dosage of 75 mg/kg fosfomycin enables serum levels of about 32 μg/ml during 4–5 h, being this level higher to the MIC of all isolated strains of S. marcescens.<