The cleaning of removable appliances was analysed in three groups: 1. cleaning with tooth-brush/toothpaste, 2. self-acting cleansing agent, 3. ultrasonic treatment. In the same patient group identical plates were fitted each for one month and cleaned by the different procedures. The microbial population and the cleansing effects were analysed by scanning electron microscopy. The findings shows clearly, that the toothbrush alone is unsatisfactory for achieving effective decontamination in the hygiene-critical areas of the plates. In contrast to this cleansing tablets and ultrasonic treatment were more effective in plaque-accumulating areas. None of the cleansing procedures effected complete decontamination of the removable appliances.