A laboratory method has been developed for studying the action of chemical disinfectants on milking equipment using a wet film and both metal and rubber surfaces. It was shown that milking machines may be soiled as effectively with fresh milk of low bacterial content as with aged milk of high bacterial content. When a single heat treatment preceded the soiling with milk and subsequent treatment with detergent and chemical disinfectant, a build-up of bacteria began on the third day. However, when the surfaces were heat treated daily 18 h before soiling, followed by cleaning and disinfection, no build-up occurred. These findings support the hypothesis that contamination of milk results from bacteria developing on the milking equipment and not merely from those added to it at each milking. The sites of bacterial contamination were shown to be the under surfaces of lid gaskets and the joints of the short milk tubes and the metal clawpiece nipples.