To increase precision in the quantitative analysis of chromosomal distribution of repeated DNA sequences detected by in situ hybridization, a number of factors important in studies of the molecular basis of chromosome polymorphism should be considered. While analysing results of hybridization, one should only use the number of grains over the sites of their regular localization, instead of those over the whole chromosome. It is also evident that to decrease variability conditioned by differences in the labelling degree of separate cells, the relative numbers of labelled grains over chromosomes should be used in the analysis and not the absolute ones. Data from those cells ought to be only included in the analysis, in which the labelling degree is sufficient to show all localization sites of the repeated sequences studied, i.e. cells should be used with comparatively constant relative numbers of labelled grains over all their regular localization sites.