Normal mammalian fibroblasts undergo a limited number of divisions when cultured in vitro before entering a state of replicative senescence. The molecular basis for the determination of the finite mitotic potential is not known. Nevertheless, simian virus 40 T antigen, among other oncogenes, is able to prevent senescence in rodent embryo fibroblasts. T antigen immortalized cells are dependent upon this protein for maintaining growth once their normal mitotic life span has elapsed. Even though the mechanism that measures the finite mitotic potential of rodent fibroblasts is not known, it has been shown that it continues to function normally in the presence of this immortalizing gene. Accumulation of cyclin-dependent kinase inhibitors such as p21Waf1/Cip1/Sdi1 could potentially be a component of the mechanism that determines the finite life span. Here we show that accumulation of p21Waf1/Cip1/Sdi1 does not correlate with this biological counting mechanism, but we have identified p24, a p21Waf1/Cip1/Sdi1-related protein, whose accumulation does correlate with the measurement of the finite proliferative potential of rodent embryo fibroblasts and suggest that sequestration might be a mechanism by which its activity is regulated.