We have studied aging at the cellular level by examining skin fibroblast cultures derived from skin biopsy samples obtained from old and young members of a longitudinal study on human aging. Results from a number of studies already completed indicate that cell cultures from old human donors, unlike cultures from young donors, have impaired cell proliferation and reduced induction of sister chromatid exchanges. A number of other parameters, however, are not affected by the age of the human fibroblast donor. These include cell protein and RNA content and the rate of cellular macromolecular synthesis. Studying cell cultures derived from members of an ongoing longitudinal study permits examination of correlations between in vivo and in vitro measurements on the same people. In addition, it allows for future longitudinal studies on human cellular aging.