Neonatal, adult, and aged donor human skin fibroblast cell cultures have been characterized for the population doubling potential and in vitro-in vivo relationship. Hybrid cells derived from individual whole-cell fusions of replicating GRC 387 (aged) and CSC 303 (neonatal) cells demonstrated that an intermediate mode of replication between that of the two parental cell lines occurred; therefore the longevity of the aged fibroblast cells was enhanced. The GRC 387 cells have an extended post-replicative phase-out period in comparison with the CSC 303 cells, and the experimental hybrids demonstrated a 20-25% increase of the period over that of the GRC 387 cells.