The purpose of this study is to examine the viability of PDL cells in rat molars by using MTT assay and to verify the MTT assay through the histologic observation. Thirty of Sprague-Dawley white female rats of 4-weeks old with a body weight of about 100 grams were used. Groupings are as follows : Immediate Group : Positive control group(n=10)-after extraction immediately. Dried Group : Negative control group(n=10)-after drying for an hour under warm dry. $ViaSpan^{circledR}$ Group : 1hour $ViaSpan^{circledR}$ group(n=10)-after storing in $ViaSpan^{circledR}{;}at{;}4^{circ}C$ for 1hour. Ten teeth of each group were treated as same as above and replanted to the original socket of experimental animals. After two weeks of replantation. all the experimental animals were sacrificed. And after fixation, extracted maxillary jaw was dimineralized. After it was embedded in paraffin. serial section by $5mu extrm{m}$ was carried out and for construction of specimen, hematoxylin-eosin dye was used. The mean MTT measurement of immediate group(positive control) is 2.81 and the mean measurement of dried group(negative control) is 0.98 which is significantt differnt(P<0.05), The mean measurement of $ViaSpan^{circledR}$ group is 2.65 and there is significant difference between dried group and $ViaSpan^{circledR}$ group(P<0.05), However, there is no difference between immediate group and $ViaSpan^{circledR}$ group. The average resorption points of immediate group is 3.03 points. In the dried group, average 6.44 points resorption and 2.68 points showed resorption in the $ViaSpan^{circledR}$ group. Unlike with MTT assay, there was no significant difference between the immediate group and $ViaSpan^{circledR}$ group. The usage of MTT assay as a viable cell marker may give us a better indication of the maintenance of periodontal ligament cell vitality.