Calcium phosphate-decorated polyester matrices are promising scaffolds for bone tissue engineering that combine the tunable degradation of synthetic polymers and the osteoconductivity of calcium phosphate minerals. In this study, scaffolds decorated with stabilized amorphous calcium phosphate (ACP) minerals—which exhibit sustained dissolution and release of calcium and phosphate ions—were tested for their ability to support osteoblast proliferation and stimulate differentiation. The two ACPs tested were zirconia-hybridized ACP (ZrACP), which releases ions over a few days and converts in aqueous solution to hydroxyapatite (HAP), and zinc-hybridized ACP (ZnACP), which has a longer period of sustained ion release. MC3T3-E1 pre-osteoblasts were cultured in these scaffolds for up to 21 days, and cell number, alkaline phosphatase (ALP) activity and expression of osteogenic and bone-specific proteins were measured. Cell number, prostaglandin E2 (PGE2) synthesis and osteopontin (OPN) mRNA expression were elevated on calcium phosphate-decorated scaffolds relative to PLGA controls, while mRNA expression of osteocalcin (OCN), bone sialoprotein and bone morphogenetic protein (BMP)-4 were suppressed. Although MC3T3-E1 responses to the two ACPs were not statistically different, ZrACP—which converts more quickly to HAP—gave rise to slightly higher levels of mRNA expression for BMP-4, osterix, vascular endothelial growth factor (VEGF)-A, OCN and OPN, but slightly lower levels of PGE2 synthesis, ALP activity and cell number. These results indicate that sintered PLGA microsphere scaffolds decorated with 0.5 wt% ZnACP or ZrACP support cell attachment and elicit a series of biological responses, but these responses do not appear to accelerate osteoblast differentiation. Copyright © 2010 John Wiley & Sons, Ltd.