Tissue nonspecific alkaline phosphatase (TNAP) knockout (ko) mice manifest defects in bone mineralization that mimic the phenotypic abnormalities of infantile hypophosphatasia. In this article, we have searched for phenotypic differences between calvarial osteoblasts and osteoclasts in wild-type (wt), heterozygous and homozygous TNAP null mice. In vitro release of45Ca from calvarial bones, with and without stimulation with parathyroid hormone (PTH), revealed no functional difference between osteoclasts from the three TNAP genotypes. Studies of primary cultures of TNAP+/+, TNAP+/−, and TNAP−/− calvarial osteoblasts revealed no differences in the rate of protein synthesis or in the expression levels of messenger RNAs (mRNAs) for osteopontin (OP), osteocalcin (OC), collagen type I, core binding factor α1 (Cbfa 1), N-cadherin, Smad 5, and Smad 7. Release of interleukin-6 (IL-6) from calvarial osteoblasts under basal conditions and after stimulation with PTH, tumor necrosis factor α (TNF-α) or IL-1β was similar in all genotypes. The amount of cyclic adenosine monophosphate (cAMP) accumulation also was comparable. However, although cultures of primary TNAP−/− osteoblasts were able to form cellular nodules as well as TNAP positive osteoblasts do, they lacked the ability to mineralize these nodules in vitro. Mineralization also was delayed in TNAP+/− osteoblast cultures compared with cultures of wt osteoblasts. Incubation with media supplemented with recombinant TNAP, but not with enzymatically inactive TNAP, restored mineralization in ko osteoblast cultures. Our data provide evidence that osteoblasts in TNAP null mice differentiate normally but are unable to initiate mineralization in vitro. The fact that even heterozygous osteoblasts show delayed mineralization provides a rationale for the presence of bone disease in carriers of hypophosphatasia.