Cellular aging of diploid rat fibroblasts in vitro occurs as the result of a three-stage differentiation sequence, taking place in the morphologically recognizable differentiating cell compartment of the fibroblast stem cell system. Simultaneously with an age-dependent morphological differentiation there is an age-dependent biochemical differentiation of the fibroblasts. In cell strains with the genetic background Lewis, the cellular differentiation state, the major histocompatibility complex composition (H-1) and the H-1-complex-controlled expression of endogenous type C viruses determine the ultimate fate of the senescent cell, leading either to cellular degeneration or to cellular neoplastic transformation.