a Calcium phosphate (CaP) based ceramics are used as bone graft substitutes in the treatment of bone defects. The physico-chemical properties of these materials determine their bioactivity, meaning that molecular and cellular responses in the body will be tuned accordingly. In a previous study, we compared two porous CaP ceramics, hydroxyapatite (HA) and b-tricalcium phosphate (TCP), which, among other properties, differ in their degradation behaviour in vitro and in vivo, and we demonstrated that the more degradable b-TCP induced more bone formation in a heterotopic model in sheep. This is correlated to in vitro data, where human bone marrow derived mesenchymal stromal cells Insight, innovation, integrationb-Tricalcium phosphate (b-TCP) and hydroxyapatite (HA), two calcium phosphate ceramics, were previously shown to induce bone formation in the muscle of dogs (after implantation without cells). b-TCP yielded more bone than HA, making it a promising bone graft substitute. Using an in vitro model, here we compared the early biological response of human mesenchymal stromal cells to these ceramics. We identified a set of genes upregulated by b-TCP compared to HA and further correlated that with the physico-chemical properties of the ceramics, where calcium ions are expected to play a role. This model may allow us to understand the complex in vivo mechanism that leads to such discrepant bone formation outcomes, allowing us to design more efficient biomaterials.