ALP is a representative enzyme of osteoblast differentiation and the ALP activity is an indicator of osteoblastic differentiation. Osteocalcin secretion is another biological marker for terminal differentiation of osteoblasts . Although the osteoblasts have similar DNA contents and total protein levels on all three samples after incubation for 4 and 9 days (Fig. 6), higher ALP activity and a larger osteocalcin content are observed from the Ag/N2 PIII surface (Fig. 7) suggesting early and mature proliferation and differentiation.