The mitochondria-binding fluorescent dye rhodamine 123 (R123) was used to sort two subpopulations of cells from mass cultures of old human fibroblasts (HF). Both subpopulations showed unusual dye retention kinetics when compared to young HF. Additionally, electron microscopy revealed differences in numerical density of mitochondria between young and old HF. Moreover, the two subpopulations exhibited differences in radiolabeled thymidine incorporation and proliferative capacity. The results obtained suggest a possible relationship between mitochondria, proliferative potential and cellular aging.