The analysis of nuclear DNA contents in various tissues of potato genotypes showed that flow cytometry is a rapid method to characterize large populations of cells for polysomaty, that is, the occurrence of cells with normal DNA levels together with cells containing endoreduplicated nuclei. The proportion of endoreduplicated nuclei varied in different tissues and genotypes of potato. The analysis of callus and cell cultures showed that the temporal changes in nuclear DNA contents during in vitro growth can be followed and the degree of polyploidization quantified. It is concluded that flow cytometry is a highly suitable method to detect ploidy changes in differentiated plant tissues and calli which are often not amenable for chromosome number determination.