Methods which have been used to determine megakaryocyte ploidy in animals and humans are reviewed. Although the number of megakaryocyte nuclear units counted in bone marrow squashes is roughly proportional to ploidy, accurate determinations of DNA content require the use of microphotometry or flow cytometry. New techniques should make it possible to distinguish polyploidizing megakaryoblasts from promegakaryocytes and mature megakaryocytes which have arrested polyploidization. Only the latter should be included in histograms of the number of endoduplications, since only those have expressed their full polyploidization potential. Statistical techniques are available for analysis and comparison of both raw ploidy distributions or histograms of endoduplication numbers.