To find out the differences between periodontal ligament cells (PDL cells) and gingival fibroblast cells (GFB cells), alkaline phosphatase, a marker enzyme for osteoblast, was used to measure the activities and ⁴⁵CaCl_² isotope was used to find out cellular and release of ⁴⁵Ca, a requisite for bone formation,. PDL cells and GFB cells from 1 to 5 passages were also measured in alkaline phosphatase activity assay. By the use of above methods, followings were concluded that the PDL cells and the GFB cells have characteristics that are different from each other. In that PDL cells showed large amount of calcium uptake and large amount of calcium release in initial stage, they seem to possess characteristics which are similar to osteoblast-like cells. 1. The PDL cells, in contrast to the gingival fibroblast, showed exceedingly high alkaline phosphatase activity which was highest at the second passage, decreasing thereon. But gingival fibroblasts cells showed no distinct differences in alkaline phosphatase activity as the passage were elapsed. 2. For both PDL cells and GF cells, the ⁴⁵Ca uptake was greatest at 2 hours period. The PDL cells showed higher measuring than GFB cells through out the whole time period. 3. Whereas the GFB cells showed slow increase of ⁴⁵Ca release as time relapsed, the PDL cells showed rapid increase of ⁴⁵Ca release.