Modern theory and experience question the reliability and suitability of sampling per judgment being the method for the evaluation of the percentage of cells in clinical cytology till now. For the reduction of this gap empirical studies were undertaken in order to receive proper estimates of blood smears. A simple method for the differential blood count is to take 30 or more strips of the width of the field of vision of the microscope perpendicular to the smearing direction as sampling units at random or by systemtaic selection with a random starting point. For the class of cells under consideration as well as for whole the cell population on the slide, the coefficient of variation of the mean of the cells within different strips has to be less than 0,1. Smears of a width of 10 mm instead of the usual 22 mm seems to be preferable. Under such conditions more than 1.000 cells are to be counted for a reliable estimate of the more frequent classes of cells. It is not supposed that this tedious method will be carried out by technicians but modern apparatus are presumed to give consistent estimates in such a way.