Abstract: A method is proposed for detecting foreign bodies in fluids to investigate the fluids such as infusion liquids for foreign bodies which can lead to the plugging of infusion needles or other dangers for a patient. In the method light is directed onto the fluid and the light departing from this fluid is detected. The fluid is illuminated evenly with polarized light, and the light penetrating through the fluid is further dimmed by further polarization. The light intensity of individual measuring regions is measured through photo-electric sensing and the measured intensity is compared with an adjustable threshold so that when the threshold is exceeded at any given time during a measuring interval, corresponding signals are produced.