Saito et al recently reported that the alkaline phosphatase (ALPase) of human periodontal ligament fibroblasts (HPLF) showed remarkably high activity which was similar, but not identical phenotype, to that present in osteoblasts, and suggested that HPLF could be termed as "osteoblastic fibroblast." The present study attempts to explore the ALPase synthesized on HPLF in relation to 1,25(OH)2D3. These HPLF were obtained by the explantation method and then subcultured in D-MEM containing 2 mg FCSP/ml, 50 micrograms ascorbic acid/ml and penicillin/streptomycin after trypsinization. The HPLF were inoculated at a cell density of 1.25 x 10(4) cells/cm2 in culture wells. After 24hr, the HPLF were treated every two days for 7 days with 0.5-10nM 1,25 (OH)2D3. Then, ALPase activity, DNA and protein contents were assayed by the methods using p-nitrophenylphosphate, diaminobenzoic acid and Coomassie Brilliant Blue, respectively. Also, ALPase was prepared from the confluent HPLF incubated with 5 nM 1,25 (OH)2D3 for 12 days, and digested with and without trypsin. The crude ALPase which was solubilized with 10mM Tris-HCl, pH 7.4 containing 0.2 mM MgCl2 and 0.1% NP-40 was applied to 5-15% gradient SDS-PAGE and stained with beta-naphththylacid phosphate and First Blue BB salt in 60 mM borate buffer pH 9.7. The cell growth which was assayed by DNA contents and the incorporation of 3H-thymidine was decreased by 1,25(OH)2D3. On the other hand, ALPase activity was increased approximately 3.6 fold at 6 day by the addition of 5 nM 1,25(OH)2D3. From the separation of ALPase activity on SDS-PAGE, 110 K and 120-130 K ALPase were identified. The 110 K ALPase, which was not changed by 1,25(OH)2D3, was converted to 100K, releasing 10K peptide after trypsin treatment. This 110K ALPase might be tightly associated with cell membrane structure. The 120-130K ALPase was remarkably increased by 1,25(OH)2D3 on SDS-PAGE and completely digested with trypsin. The ALPase in the cultured HPLF might be located not only on the plasma membrane but also in the extracellular matrix. Therefore, 1,25(OH)2D3 may regulate the cell cycle and also the gene expression of ALPase of HPLF.