Fosfomycin has been shown to ameliorate the dose-limiting ototoxicity and nephrotoxicity associated with the antineoplastic agent cis-platinum. Clinical use of fosfomycin in cis-platinum-treated tumor patients critically depends on retaining therapeutic efficacy of cis-platinum while designing modalities to reduce side effects. This study uses two independent assay systems for cell viability and proliferation; we demonstrate that cytocidal activity of cis-platinum against human osteosarcoma cell lines is not affected by various concentrations of fosfomycin up to 0.1 mg/ml in vitro. This suggests that in these cells there is no direct pharmacodynamic interaction of cis-platinum and fosfomycin with respect to mechanisms crucial for tumor kill.