The purpose of this study was to investigate the effect of sintering different calcium phosphate (CaP) surfaces on protein adsorption and osteoblast cell response. As-received and sintered hydroxyapatite (HA) and brushite were used in this study. X-ray diffraction revealed a poorly crystallized HA structure for the unsintered HA and a highly crystallized HA for the sintered HA surfaces. A brushite-type structure was indicated on the unsintered brushite surfaces, whereas sintered brushite surfaces contained mixtures of different CaP phases. Using 1 mg/mL albumin solution, protein was suggested to selectively adsorb on the CaP surfaces. A statistically higher albumin adsorption was observed on unsintered HA (9.5 μg/mL) and unsintered brushite (50.1 μg/mL) surfaces compared to sintered HA (3.2 μg/mL) and sintered brushite (3.4 μg/mL) surfaces. In the in vitro study using osteoblast cells, no statistical responses were observed between cells cultured on sintered HA and sintered brushite after 8 days of incubation. However, statistical differences in osteocalcin and protein production were observed between the unsintered HA and unsintered brushite. In addition, statistical differences in protein production, alkaline phosphatase activity, and osteocalcin production were observed between sintered CaP and unsintered CaP surfaces. From the protein adsorption and cell responses observed in this study, it was concluded that CaP surfaces need to be fully characterized prior to implantation.