This work has proposed a new theoretical approach to analysis of histograms of DNA content, which are obtained by the method of flow cytometry, in cells of Drosophila melanogaster imaginal discs. The precision of measurements of the DNA amount in G1 and G2(M) phases has been shown to be limited by precision of instrument tuning of zero of the flow cytometer. Use of the calculative zero of the flow cytometer and of dividing cells as standards of the DNA content is able to increase severalfold the precision of the DNA measurements in nuclei of the species. Comparative analysis of errors of various methods of measurement of the DNA content in cell nuclei is also performed. For methods of flow fluorescent cytometry, confocal scanning, and cytophotometry of the Feulgen-stained nuclei, it has been shown that, at present, the mean square errors of the DNA content measurements are within the interval of values considered acceptable for biological studies (0.02 < CV < 0.06).