A comparative study was made of the penetration of fosfomycin, penicillin G, ampicillin and chloramphenicol into the cerebrospinal fluid of patients with meningitis treated with combinations of fosfomycin and one of the other three antibiotics. Minimal inhibitory concentrations and in vitro interaction of these antibiotics against Streptococcus pneumoniae and Neisseria meningitidis strains were determined. 90–96.5% of these strains were sensitive to penicillin G, 95–96.5% to ampicillin, 85–100% to chloramphenicol and 90–100% to fosfomycin. Fosfomycin shows a more marked synergism with penicillin G or ampicillin than with chloramphenicol against both bacterial species. The percentages of penetration into the cerebrospinal fluid were: chloramphenicol, 32%; fosfomycin, 25.7%; ampicillin, 15.9%, and penicillin G, 7.9%. The clinical results show that the combination of fosfomycin + penicillin G or fosfomycin + ampicillin can be an alternative in the treatment of meningitis produced by moderately susceptible strains of S. pneumoniae and N. meningitidis to penicillin G and ampicillin.