the invention relates to a method for the examination of cells in a culture medium , in particular for in - situ microscopy in a bio - reactor , whereby cells in a sample volume , the depth of which is defined by windows in the direction of the optical axis of the microscope , are microscopically imaged and are automatically recorded and processed by means of an image processing system . said method is characterized in that the depth of the sample volume is adjusted to the size of the cells by successively reducing the separation of the windows while the image size of the cells is simultaneously verified by the image processing system such that a separation value is determined at which the image size of the cells begins to grow , thus corresponding to flattening caused by the contact pressure of the windows , and that the separation of the windows is set to said separation value for the examination .