The invention relates to a method for the examination of cells (20, 38) in a culture medium, in particular for in-situ microscopy in a bio-reactor, whereby cells in a sample volume, the depth (d) of which is defined by windows (14, 16) in the direction of the optical axis of the microscope (18), are microscopically imaged and are automatically recorded and processed by means of an image processing system (30). Said method is characterized in that the depth (d) of the sample volume (12) is adjusted to the size of the cells (20, 38) by successively reducing the separation (d) of the windows while the image size (G) of the cells is simultaneously verified by the image processing system (30) such that a separation value (D) is determined at which the image size (G) of the cells begins to grow, thus corresponding to flattening caused by the contact pressure of the windows (14, 16), and that the separation (d) of the windows (14, 16) is set to said separation value (D) for the examination.