Summary: The aim of this study was to investigate the in vitro cytotoxieity of polyphosphoester polymer used as a novel injectable alveolar bone substitutes for controlled delivery of tetracycline. Cell culture medium was exposed to the polymer (0.01-10 mg/mL) for 24h. The L-929 mouse fibroblasts were then exposed to the treated cell culture medium for 24h. Finally, cell viability and growth were assessed by using MTT assay and Alamar Blue assay. No significant cytotoxicity of the polyphosphoester against L-929 mouse fibroblasts was observed at a concentration up to 10 mg/mL (P0.05). The two evaluation methods showed no significant differences (P0.05). This study suggests that polyphosphoester does not demonstrate any significant toxic effects to cells in vitro and has the potential to be used both as a medical device and as scaffolds in tissue engineering applications.