Passage cultures of human keratinocytes were conducted in the presence or absence of H(2)O(2), and the life-span was correlated with intracellular and extracellular state variables. The maximum numbers of population doublings (PD(m)), determined by applying the modified Gompertz equation to the time profiles of population doublings (PD), were 12.8 and 15.6 in the presence and absence of H(2)O(2), respectively. These results showed that the addition of H(2)O(2) shortened the lifespan of human keratinocytes. The value of PD(m)-PD* = 2.7 perpendicular to 0.3 was proposed as a threshold for substantial change in average cell area, irrespective of the culture conditions, along with the profiles of the remaining number of population doublings (PD(m) - PD). Furthermore, the gene expressions of cat, sod2 and sod3 tended to be upregulated when the remaining number of population doublings fell below PD(m)-PD* = 2.7 +/- 0.3.