1. A method for segmenting stained leukocytes in a blood smear, which comprises: defining at least three pixel classes, including erythrocytes, leukocytes, and image background; allocating pixels of a digital image taken of the blood smear to one of the at least three pixel classes in accordance with a chromaticity value thereof; subjecting the chromaticity values of all pixels of the image to a common transformation, upon which the pixels allocated to the class of image background appear substantially white; transforming the color image to an alternative color space representing a hue, a color saturation, and a color intensity separately, and ascertaining the hue, the color saturation, and the color intensity of all pixels; calculating a probability value for each pixel for an association thereof to a leukocyte, the probablity value corresponding to a product of a probability value for the hue of the leukocyte nucleus with at least one further probability value, and selecting the at least one further probability value from the group of probability values consisting of a probability value for a non-association of a pixel to an erythrocyte hue, or a probability value allocated to a color saturation value of each pixel, or a probability value allocated to a color intensity value or the luminosity of each pixel; defining the probability values with reference to previously ascertained and defined associations; and considering pixels with a high value for the product of the probability value as representing a leukocyte.