By impulse cytophotometry (ICP), the distribution of the frequency of cell nuclei as a function of the fluorescence intensity can be determined. The interpretation of the curves is only possible on the basis of histophysical and histochemical features. The following problems are discussed: specifity of staining with ethidium bromide, pretreatment, detritus and comparison with other independent methods. The 2nd peak of the karyogram consists of G2 cells and polyploidy of tetraploid cells. This peak is often high in cell suspensions of human malignant tumors. Polyploidy up to 16 c is sometimes noted. The stem line concept is discussed. ICP is a valuable aid in exfoliative cytology, but is not a screening method.