The unsaturated polyphosphoester (UPPE) polymer is being investigated as an injectable and biodegradable system for alveolar bone repair in the treatment of periodontal diseases. The incorporation of beta-tricalcium phosphate (beta-TCP) particles into the UPPE polymer was previously shown to significantly increase the material's mechanical properties. Moreover, in vitro experiments demonstrated that the UPPE/beta-TCP composite was capable of zero-order release of tetracycline for over 2 weeks. In this study, we investigated the in vitro cytotoxicity of each individual component, the resulting cross-linked network and the degradation products of the UPPE/beta-TCP composite using an AlamarBlue viability assay. We confirmed that each individual component except beta-TCP and the in vitro degradation products of the composite displayed a dose-dependent cytotoxic response. Once cross-linked, however, the composite did not demonstrate an adverse response. Our results suggest that the UPPE/beta-TCP composite holds great promise for use as an injectable and biodegradable alveolar bone substitute.