In serial cultures of keratinocytes isolated from human neonatal foreskin, the potential for cell attachment and cell division decreased with an increase in the number of culture passages. The specific growth rate of the cells (mu) reduced gradually with elapsed culture time and an appreciable drop in the rate was observed below mu=0.02 h(-1) owing to cellular senescence. Then, a critical point in the serial cultures was determined as the last data point keeping the mu value at more than 0.02 h(-1). In serial cultures of keratinocytes isolated from the breast skin of 22 and 70-year-old individuals, the time profiles of the mu value displayed critical points, as observed in the neonatal foreskin cultures. Plots of the mu value and average cell area against the remaining numbers of population doublings (N(dF)/t-N(d)) were found to give a relationship which overlapped irrespective of the three kinds of keratinocytes, and the propensity for a depression of growth potential [estimated as a ratio of differential value of mu to that of (N(dF)-N(d))] was much higher after the critical point than before. In addition, a data map in terms of the specific growth rate and average cell area, which could be classified into proliferative, senescent and transitional phases, was proposed for the discrimination of cellular senescence in serial cultures for the production of epithelial sheets.