Two different bone cell populations were isolated from trabecular surface of long bones of 18-day old chick embryos. One population (Pop.1) was supposed to be undifferentiated osteoblast and the other (Pop.2) was supposed to be rich in active osteoblast on the basis of following reasons: (1) Pop.1 had high replicative capacity and ALPase activity. (2) Pop.1 decreased ALPase activity when endogenous prostaglandin synthesis was inhibited by indomethacin and this inhibition was reversed by the addition of prostaglandin E_2. (3) Pop.2 had a higher ALPase activity than the cells isolated from periosteum-free calvaria which are generally thought to be osteoblastic cells, and had enhanced ALPase activity in response to prostaglandin E₂.

ALPase activities in both cell populations were dependent on their cell densities. The optimum ALPase activity was observed when the cell density was 3.0-4.0μg DNA per 2cm² of culture well in both cell populations.