A method is described by which a numerical value can be assigned to the amount of DNA-synthesis shown graphically by population-histograms obtained by Feulgen cytophotometry. The index appeared to give a reasonable measure of DNA-synthetic activity in populations having very low, moderate and high mitotic indices and closely followed labelling indices obtained by autoradiography. Thus the advantage of a numerical value is that the DNA-synthetic activities in different populations can be compared for statistical analysis.