Authors concluded an experimental study in order to find out what the effects of parathyroid hormone on cells derived from human Trabecular bone in vitro are on terms of proliferation of cells, adenylate cyclase activity and production of small substances such as osteocalcin and collagen type I. The results were as follows; 1. 3H-thymidine incorporation into cultured osteoblast was initially low (day 1–2), increased by exponential curve from day 3 till day 11. The uptakes of 3H-thymidine by osteoblasts, when bovine parathyroid hormone was added, increased dose-dependently. Greater increments were seen at concentrations of bovine PTH higher than 5.0 X10-8 mole. 2. Intracellular c-AMP accumulation was stimulated by bovine PTH in a dose-dependent manner and greater production was seen at concentrations of bovine PHT higher than 5.0 X10-8 mole. Maximal stimulation was observed at 1 X10-7 mole of bovine PTH. The concentrations of c-AMP were observed significantly higher in the presence of bovine PTH in a dose-dependent manner when compared to those of bovine PTH-absent culture condition. 3. Treatment with bovine parathyroid hormone of cultured osteoblasts resulted in increase of alkaline phophatase activity and synthesis of osteocalcin in the cultured media in a dose-dependent manner. Greater productions were observed at concentrations of bovine PTH higher than 5.0 X10-8 mole. 4. Synthesis of type I collagen by cultured osteoblasts in bovine PTH-added media was inversely proportional to their concentration, whereas control group showed minimal increments of no significance. These in vitro findings may suggest that PTH has a stimulatory effect on proliferation of osteoblast, c-AMP production and alkaline phosphatase activity in a direct manner.