Labeling index and total grain count data of periodontal ligament cells after either continuous infusion or single injection with tritiated thymidine were compared with values predicted by two different models of cell proliferation. One model assumes that all proliferating cells are homogeneous and proceed through the cell cycle at a rate described by an exponential distribution. In the second model, it is assumed that cell division is asymmetric. Mitosis produces one proliferating and one differentiated cell, each with a characteristic and distinct lifetime and proliferative capacity. The predictions of the first model deviated widely from the continuous labeling and total grain count data. In comparison, the second model predicted a linear relationship between labeling index and time which is consistent with data from mouse, hamster and rat periodontal ligament cells. The model also accurately predicted the total grain count after a single injection of tritiated thymidine. The data suggest that the periodontal ligament contains renewing and differentiated populations of cells which can be described in terms of proliferative potential and lifetime.