Human adult fibroblasts from the papillary layer of the skin showed increased survival in vitro when compared to reticular fibroblasts from the same biopsy. Fibroblasts of the papillary layer were also analysed in vitro to study the kinetics of proliferation between subcultivation and resting phase. It was found that the growth fraction and maximum cell densities vary as a function of the initial inoculum. The results are compatible with the presence of two cell populations with different growth potentials. The heterogeneity of these populations has to be taken into account when studying cellular aging.