This chapter describes the isolation, culture and staining of primary osteoblasts from the calvaria and long bones of neonatal rats. The key advantages of this assay are that it allows direct measurement of bone matrix deposition and mineralisation, as well as yielding good quantities of osteoblasts at defined stages of differentiation for molecular and histological analysis. A special focus of this chapter is on the role of β-glycerophosphate in cell-mediated mineralisation in these cultures.